Selection  and   Cross-breeding  in   Relation 

to  the  Inheritance  of  Coat-pigments 

and  Coat-patterns  in  Rats  and 

Guinea-pigs 


BY 


HANSFORD  MAcCURDY  AND  W.  E.  CASTLE 


WASHINGTON,  D.  C.  ! 

PUBLISHED  BY  THE  CARNEGIE  INSTITUTION  OF  WASHINGTON 
MAY,   1907 


Selection  and   Cross-breeding  in   Relation 

to  the  Inheritance  of  Coat-pigments 

and  Coat-patterns  in  Rats  and 

Guinea-pigs 


BY 


HANSFORD  MAcCURDY  AND  W.  E.  CASTLE 


WASHINGTON,  D.  C.  I 

PUBLISHED  BY  THE  CARNEGIE  INSTITUTION  OF  WASHINGTON 
MAY,   1907 


CARNEGIE  INSTITUTION  OF  WASHINGTON  PUBLICATION  No.  70. 


PAPERS  OF  THE  STATION  FOR  EXPERIMENTAL  EVOLUTION,  No.  8. 


CONTRIBUTIONS  FROM  THE  ZOOLOGICAL  LABORATORY  OF  THE  MUSEUM  OF 

COMPARATIVE  ZOOLOGY  AT  HARVARD  COLLEGE. 

E.  L.  MARK,  DIRECTOR.     No.  188. 


PRESS  OF  GIBSON  BROS. 
WASHINGTON,  D.  C. 


CONTENTS. 

Page. 

Continuous  versus  discontinuous  variations  as  factors  in  evolution      ....  i 

Variations  of  rats  in  coat-color  and  coat-pattern 4 

Mendelian  inheritance  of  coat-colors  and  coat-patterns  in  rats 7 

Albinism  recessive  in  relation  to  all  types  of  pigmentation 7 

Latent  pigment  characters  and  coat-patterns 8 

Modification  of  hooded  pattern  by  crossing  with  Irish 1 1 

Modification  of  hooded  pattern  by  selection 14 

Selection  for  reduced  stripe 14 

Selection  for  stripe  of  increased  size 17 

Individual  spots  of  guinea-pigs  not  unit  characters 1 8 

Color-patterns  selected 1 8 

Dutch-marked  series  (Series  D) 20 

Head -spot  series  (Series  //) 23 

Statistical  analysis  of  the  data  for  Series  D  and  // 25 

Nose-spot  series  (Series  N) 26 

Conclusions 32 

Tables 34 

Bibliography 50 

in 


SELECTION  AND  CROSS-BREEDING  IN  RELATION  TO 
THE  INHERITANCE  OF  COAT-PIGMENTS  AND  COAT- 
PATTERNS  IN  RATS  AND  GUINEA-PIGS. 


BY  HANSFORD  MACCURDY  AND  W.  E. 


CONTINUOUS  VERSUS  DISCONTINUOUS  VARIATIONS  AS 
FACTORS  IN  EVOLUTION. 

It  is  generally  agreed  that  the  course  of  evolution  is  largely  influenced 
by  two  factors,  variation  and  heredity;  but  opinions  differ  as  to  what  sorts 
of  variation  have  evolutionary  significance  and  as  to  the  manner  of  their 
inheritance. 

It  has  been  recognized  by  several  investigators  that  variations  are  of 
two  distinct  sorts.  Bateson  has  called  these  two  sorts  of  variation  contin- 
uous and  discontinuous ;  more  recently  De  Vries  has  called  them  fluctuations 
and  mutations,  respectively. 

By  continuous  variation  (or  fluctuation)  we  understand  ordinary  indi- 
vidual variation  within  a  species.  The  individuals  differ  among  themselves 
in  size,  color,  and  other  structural  features.  By  examining  a  considerable 
number  of  them  we  can  form  an  idea  of  what  is  the  commonest  (or  modal) 
condition  as  regards  each  structural  feature ;  and  likewise  what  is  the  average 
(or  mean)  condition. 

Usually,  but  not  always,  the  modal  and  mean  conditions  are  approximately 
the  same,  and  any  other  condition  is  the  less  frequent  in  occurrence,  the 
greater  its  deviation  from  them.  It  follows  that  the  most  extreme  condi- 
tion observed  is  connected  with  the  most  usual  (or  modal)  condition  by 
an  unbroken  series  of  intermediate  conditions,  and  we  may  call  the  series 
as  a  whole  "continuous."  The  distribution  of  the  individuals  in  such  a 
series  is  governed  by  the  laws  of  "chance,"  and  may  be  successfully 
analyzed  by  statistical  methods. 

We  commonly  think  of  a  "chance"  result  as  something  entirely  beyond 
the  control  of  law,  but  in  reality  such  is  not  the  case.  Nothing  is  beyond 
the  control  of  law.  If  a  blindfolded  person  puts  his  hand  into  an  urn  con- 
taining a  mixture  of  black  and  of  white  balls,  it  is  a  matter  of  chance  whether 
he  grasps  a  black  or  a  white  ball;  but  if  he  repeats  the  operation  a  con- 
siderable number  of  times,  it  is  perfectly  certain  that  he  will  draw  balls  of 

i 


2  INHERITANCE   OF  COAT-PIGMENTS  AND  COAT-PATTERNS 

both  sorts  in  approximately  the  same  proportions  in  which  they  occur  in 
the  jar.  The  result  is  a  "chance"  one,  but  controlled  by  a  perfectly  definite 
mathematical  law. 

A  "chance  result"  has  been  aptly  defined  as  the  result  of  a  number  of 
causes  acting  independently  of  each  other.  If  this  is  a  valid  definition, 
then  a  continuous  series  of  variations  is  due  to  no  single  cause  but  to  several 
mutually  independent  ones.  Some  of  the  causes  may  be  external  in  origin, 
others  internal;  some  temporary  in  their  action,  others  permanent.  It 
should  not  surprise  us,  therefore,  to  find  that  continuous  variations  differ 
greatly  in  the  degree  of  their  inheritance.  De  Vries,  indeed,  has  maintained 
that  they  are  not  inherited  at  all,  except  temporarily;  that  selection  of 
abmodal  variations  from  a  continuous  series  is  unable  permanently  to  modify 
a  race ;  that  the  modifications  will  persist  only  so  long  as  selection  continues, 
but  will  speedily  disappear  when  selection  is  arrested.  This  conclusion, 
however,  seems  to  us  altogether  too  sweeping.  A  priori  there  is  no  reason 
to  suppose  that  all  the  causes  operative  to  produce  continuous  variation 
are  external  in  origin  and  temporary  in  action,  as  De  Vries's  conclusion 
would  seem  to  imply.  If  there  are  in  operation,  in  the  production  of  a 
continuous  series  of  variations,  causes  internal  in  origin,  resident  in  the 
constitution  of  the  germinal  substance,  so  much  of  the  result  as  is  due  to 
those  causes  should  be  inherited  and  so  should  be  permanent.  De  Vries,  we 
believe,  has  overlooked  this  factor  entering  into  the  problem.  He  has 
assumed  that  all  the  causes  of  continuous  variation  ("fluctuations")  are 
either  external  in  origin  or  due  to  conditions  of  the  germinal  substance 
purely  temporary.  He  holds,  we  believe  rightly,  that  all  inheritance  is  due 
to  germinal  modification ;  but  assumes,  we  believe  without  sufficient  warrant, 
that  permanent  germinal  modification  is  not  a  factor  in  the  production  of 
fluctuations. 

Another  category  of  variations,  discontinuous  variations  (which  include 
the  mutations  of  De  Vries),  is  considered  by  Bateson  and  De  Vries  as  the 
true  and  only  expression  of  permanent  germinal  modification.  But,  grant- 
ing the  truly  germinal  origin  of  mutations,  it  does  not  follow  that  they  are 
the  only  product  of  germinal  modification. 

A  discontinuous  variation,  as  the  name  suggests,  is  unconnected  by  inter- 
mediate conditions  with  the  usual  (modal)  condition  of  the  species.  It 
represents  a  change,  more  or  less  abrupt,  from  the  modal  condition  of  the 
species,  and  is  strongly  inherited,  a  fact  which  indicates  clearly  its  exclu- 
sively germinal  origin. 

In  the  category  of  discontinuous  variations  belong  abrupt  changes  in 
pigmentation  and  hairiness  among  both  animals  and  plants,  changes  in  the 
number  of  digits  or  of  the  number  of  phalanges  in  a  digit  among  vertebrates, 
in  the  presence  or  absence  of  horns  among  animals  and  spines  among  plants, 
and  other  similar  conditions. 


IN  RATS  AND  GUINEA-PIGS.  3 

Such  changes  are  not  the  result  of  selection;  they  often  appear,  as  it 
seems,  spontaneously,  and  they  are  permanent  in  the  race,  if  isolated. 

De  Vries  maintains  that  all  species-forming  variations  are  of  this  sort; 
that  selection  is  unable  to  form  new  species,  because  it  can  neither  call  into 
existence  mutations  nor  permanently  modify  a  race  by  cumulation  of 
abmodal  fluctuations.  Darwin,  on  the  other  hand,  and  the  great  majority 
of  his  followers,  while  admitting  that  races  are  occasionally  produced  by 
discontinuous  or  "sport"  variation,  ascribe  evolutionary  progress  chiefly  to 
the  cumulation  through  long  periods  of  time  of  slight  individual  differences, 
such  as  De  Vries  calls  fluctuations.  The  issue  between  the  two  views  is 
sharp  and  clear.  According  to  De  Vries,  if  we  rightly  understand  him, 
selection  is  not  a  factor  in  the  production  of  new  species,  but  only  in  their 
perpetuation,  since  it  determines  merely  what  species  shall  survive;  accord- 
ing to  the  Darwinian  view,  new  species  arise  through  the  direct  agency 
of  selection,  which  leads  to  the  cumulation  of  fluctuating  variations  of  a 
particular  sort. 

De  Vries  and  the  Darwinians  differ  not  only  as  to  the  part  which  selection 
plays  in  evolution,  but  also  as  to  the  nature  of  the  material  upon  which 
selection  acts.  According  to  De  Vries,  species  are  not  modified  by  selection; 
mutations  are  new  species  and  selection  determines  only  what  mutations 
shall  survive,  fluctuations  having  no  evolutionary  significance.  On  the 
Darwinian  view,  all  species,  whether  arising  by  mutation  or  not,  are  subject 
to  modification  by  selection. 

A  great  deal  can  be  said  in  favor  of  each  of  these  contrasted  views,  but  dis- 
cussion is  at  present  less  needed  than  experimental  tests  of  the  views  outlined. 
To  De  Vries  we  owe  much  for  showing  that  such  tests  are  possible. 

It  was  our  purpose  to  make  tests  of  this  sort  when  we  undertook  the 
experiments  described  in  this  paper.  The  questions  to  which  principally 
attention  has  been  directed  are  these:  (i)  Can  discontinuous  variations  be 
modified  by  selection  alone?  (2)  Can  discontinuous  variations  be  modified 
by  cross-breeding?  A  negative  answer  to  these  questions  will  support  the 
view  of  De  Vries ;  an  affirmative  answer  will  support  the  Darwinian  view, 
because  it  will  show  that  through  selection  new  conditions  of  organic  stability 
can  be  obtained;  that  is,  new  species  may  be  produced. 

The  material  used  consisted  of  certain  discontinuous  variations  in  the 
color-pattern  of  rats.  The  general  result  obtained  is  this:  Various  color- 
patterns,  like  the  several  pigments  found  in  the  rodent  coat,  are  mutually 
alternative  in  heredity.  Each  group  of  individuals  referred  to  the  same 
type  of  color-pattern  forms  a  continuous  series  fluctuating  in  accordance 
with  the  laws  of  chance  about  a  common  modal  condition.  The  different 
types  in  general  do  not  overlap ;  they  form  a  discontinuous  series.  Now,  these 
types  may  be  modified  in  two  different  ways :  (a)  By  selection  of  abmodal 


4  INHERITANCE  OF  COAT-PIGMENTS  AND  COAT-PATTERNS 

variates  within  the  same  continuous  series,  and  (6)  by  cross-breeding  between 
different  types.  There  is  no  evidence  that  one  of  these  methods  has  effects 
less  permanent  than  the  other.  So  far,  then,  as  these  experiments  go,  they 
support  the  Darwinian  view  rather  than  that  of  De  Vries. 

VARIATIONS   OF   RATS   IN    COAT-COLOR   AND    COAT-PATTERN. 

Variations  in  the  pigmented  coat  of  rodents  are  of  two  principal  sorts: 
(i)  Variations  in  the  character  of  the  pigments  found  in  the  coat,  and  (2) 
variations  in  the  distribution  of  those  pigments.  The  character  of  the 
pigmentation  in  the  wild  rodent  is  nearly  always  complex.  Two  or  three 
pigments  are  associated  together  in  the  same  hair,  but  they  differ  in  their 
regional  distribution,  so  that  a  grayish  or  brown  "ticked"  coat  results,  incon- 
spicuous against  many  backgrounds.  The  coat  of  the  house-mouse  (Bate- 
son,  103)  and  that  of  the  wild  guinea-pig  (Castle,  105)  contain  three  optically 
different  pigments — yellow,  brown,  and  black.  These  all  coexist  in  the 
same  hair.  In  certain  fancy  varieties  of  these  rodents,  a  single  pigment  is 
present  without  the  others,  or  the  distribution  of  the  pigments  is  such  that 
only  one  sort  is  conspicuous.  Animals  pigmented  thus  are  known  as  black, 
chocolate,  and  yellow  (or  red)  varieties.  If  all  three  pigments  are  absent 
from  the  coat  and  likewise  from  the  retina  of  the  eye,  a  condition  known  as 
total  albinism  obtains. 

In  rats,  rabbits,  and  certain  other  rodents,  black  and  yellow  self- varieties 
are  well  known,  but  no  pure  chocolate  animal  has  been  observed. 

Total  albinism  and  the  several  "self"  conditions  of  pigmentation  are  all 
mutually  alternative  in  inheritance. 

Variations  in  pigment  distribution  on  the  body  result  commonly  either 
from  entire  absence  of  pigment  from  certain  regions  of  the  body,  in  which 
case  the  coat  has  white  markings,  or  from  the  occurrence  of  different  pig- 
ments singly  in  different  body-regions,  in  which  case  the  body  bears  spots  of 
different  colors.  Both  sorts  of  variation  may  occur  simultaneously,  in  which 
case  the  body  is  spotted  with  pigments  of  different  sorts  and  with  white. 

The  color-varieties  of  rats  are  fewer  and  simpler  than  those  of  mice,  rab- 
bits, and  guinea-pigs.  Aside  from  albinos,  there  are  only  two  "self"  (i.  e., 
uniformly  colored)  varieties,  namely,  gray  (or  brown,  the  color  of  the  wild 
Mus  decumanus)  and  black.  Gray  is  a  Mendelian  dominant  in  relation  to 
black. 

As  regards  coat-pattern,  there  occur  two  conditions  of  partial  albinism, 
which  differ  from  each  other  only  in  degree,  but  which  may  be  obtained 
each  probably  in  a  pure  (homozygous)  condition.  These  two  patterns  may  be 
called  "Irish"  and  "hooded."  Bach  occurs  either  with  gray  or  with  black 
pigmentation.  The  "Irish"  of  fanciers,  as  described  by  Doncaster  (:o6), 


PLATE  1 


VARIATION    IN    EXTENT    OF    PIGMENTATION    AMONG    CROSS-BRED 
HOODED    RATS. 

The  usual  condition  in  the  original  stock  is  shown   in   Fig.  4,   reduced   pigmen- 
tation in  Figs.  I  and  3,  and  increased  pigmentation  in  Fig.  2. 


THE    MERIOEN   GRAVURE   CO. 


IN  RATS  AND  GUINEA-PIGS.  5 

applies  only  to  black-pigmented  individuals,  but  for  lack  of  a  better  descrip- 
tive term,  we  shall  apply  it  to  all  animals  having  the  color-pattern  of 
"Irish"  rats,  whether  gray  or  black  pigmented.  A  rat  of  the.  Irish  pattern 
has  pigmented  sides  and  dorsal  surface,  but  bears  more  or  less  white  fur 
upon  its  belly,  varying  in  extent  from  a  few  white  hairs  midway  between 
the  front  legs  to  a  wholly  white  ventral  surface. 

Doncaster  (:o6)  has  distinguished  two  types  of  Irish  rats,  in  one  of  which 
the  white  is  more  extensive  than  in  the  other.  He  has  observed  that  the 
rats  with  larger  white  areas  are  regularly  heterozygous,  producing  hooded 
as  well  as  Irish  offspring  when  mated  inter  se.  Our  experiments  in  the  main 
confirm  this  idea.  It  is  not  possible  to  determine  with  certainty,  from  the 
size  of  the  ventral  patch  alone,  whether  a  particular  Irish  rat  does  or  does 
not  contain  the  hooded  pattern  in  a  recessive  condition,  but  in  a  lot  of  Irish 
rats  of  similar  ancestry  those  with  the  larger  white  patch  oftener  transmit 
the  hooded  condition,  while  those  which  do  not  transmit  the  hooded  condi- 
tion oftener  have  a  small  white  patch. 

In  hooded  rats  (pi.  i,  figs.  1-4)  the  white  areas  are  more  extensive  than 
in  Irish  ones ;  pigment  occurs  only  on  the  head,  shoulders,  and  forelegs  (con- 
stituting the  "hood"  of  fanciers),  and  as  a  median  dorsal  stripe  extending 
back  on  to  the  tail,  the  stripe  being  sometimes  continuous,  sometimes  inter- 
rupted, and  of  varying  width.  According  to  the  nature  of  the  pigment 
which  they  bear,  hooded  rats  may  be  distinguished  as  gray  hooded  or  as 
black  hooded,  precisely  as  animals  bearing  the  Irish  pattern  are  designated 
either  gray  Irish  or  black  Irish. 

When  crosses  are  made  between  rats  differing  in  color-pattern,  the  more 
extensively  pigmented  pattern  tends  to  dominate  in  the  offspring,  a  fact 
recognized  by  Crampe  ('77-84,  '85),  Bateson  (:o3)  and  Doncaster  (:o6). 
The  dominance,  however,  is  not  complete,  so  that  the  result  might  be  de- 
scribed as  "goneoclinic,"  i.  e.,  intermediate  between  the  parental  forms  but 
approximating  one  much  more  nearly  than  the  other,  in  this  case  always 
the  more  heavily  pigmented  one.  Thus,  a  cross  between  a  wild  gray  male 
rat  and  a  black  hooded  female,  known  to  be  homozygous,  produced  a  litter 
of  seven  young,  all  gray,  but  with  a  small  patch  of  white  on  the  chest,  vary- 
ing in  extent  from  merely  a  few  white  hairs  to  an  area  of  4  to  5  sq.  cm.  The 
same  wild  male,  mated  with  an  albino  female,  produced  a  litter  of  young 
similar  in  character,  all  with  some  white  below.  Bateson  (103,  p.  78,  foot- 
note) mentions  a  similar  result  obtained  by  Miss  Douglas. 

Again,  a  cross  between  an  Irish  and  a  hooded  individual  produces  Irish 
offspring.  An  example  will  be  found  on  page  n  in  the  matings  of  black 
Irish  $141  with  black-hooded  males,  producing  nineteen  offspring,  all  Irish. 
Most  matings  of  Irish  with  hooded  rats  have  in  our  experiments  produced 
offspring  of  both  sorts,  not  because  of  reversal  in  the  nature  of  the  domi- 


6  INHERITANCE  OF  COAT-PIGMENTS  AND  COAT-PATTERNS 

nance,  but  because  the  majority  of  the  Irish  rats  used  happen  to  have  been 
heterozygous,  bearing  the  hooded  pattern  as  a  recessive  character;  but,  as 
we  have  seen,  this  is  not  a  necessary  relationship.  Hooded  rats,  however, 
bear  no  other  color-pattern,  so  that  any  pair  of  hooded  rats  of  whatever 
pedigree  will  produce  only  hooded  pigmented  offspring. 

To  sum  up  in  a  brief  statement  the  principal  facts  presented  concerning 
the  coat-patterns  of  rats,  the  following  series  of  conditions  may  be  recog- 
nized, each  with  pigmentation  less  extensive  than  the  preceding:  (i)  Self, 
whole  body  pigmented;  (2)  Irish,  whole  body  pigmented  except  more  or 
less  of  the  ventral  surface ;  (3)  Hooded,  only  the  head,  shoulders,  and  usually 
a  median  dorsal  stripe  pigmented;  (4)  Albino,  no  pigmentation. 

Each  condition  behaves  as  a  dominant  toward  those  which  follow  it  in 
the  series,  and  as  a  recessive  toward  those  which  precede  it.  Bateson  (103, 
p.  81)  refers  to  Crampe's  failure  to  obtain  the  self  or  the  albino  conditions 
by  selection  from  the  parti -colored,  and  adds:  "The  types  are  in  fact  definite, 
and  can  not  be  built  up  by  cumulative  selection."  The  statement  applies 
strictly  only  to  the  two  extremes  of  the  series,  viz,  self  and  albino,  but  by 
implication  to  the  others  also.  Our  experiments,  however,  indicate  that  it 
is  possible  to  modify  by  selection  and  cross-breeding  both  the  Irish  and  the 
hooded  conditions,  leading  to  the  production  of  intermediate  conditions. 
We  suspect  that  the  same  may  be  true  of  the  self  condition  also.  It  has 
been  pointed  out  elsewhere  (Castle  and  Forbes,  :o6;  Castle,  :o6),  that  the 
theoretical  absolute  gametic  purity,  in  Mendelian  inheritance,  probably  does 
not  exist  in  any  case.  Heterozygosis  leads  inevitably  to  modification  in 
character  of  the  conjugating  gametes,  not  simply  as  regards  the  entire 
assemblage  of  characters,  but  as  regards  each  character  considered  sepa- 
rately. The  degree  of  modification  is  probably  indicated  roughly  by  the 
imperfection  of  dominance  in  the  heterozygote.  Thus,  when  a  perfect 
blend  is  obtained,  as  in  the  inheritance  of  ear-length  in  rabbits,  the  gametes 
transmit  that  intermediate  character.  But  when  dominance  is  very  com- 
plete, as  in  a  cross  between  a  self  and  an  albino  mammal,  the  segregation  of 
self  and  albino  conditions  is  very  complete  among  the  gametes  formed  by 
the  cross-breeds.  Yet  when  spotted  individuals  result  from  cross-breeding 
between  self  and  albinos,  these  spotted  individuals  form  gametes  which 
transmit  that  same  mosaic  character.  Now,  the  cross  between  self  and 
albino  rats  gives  rats  of  Irish  pattern,  but  quite  variable.  But  our  experi- 
ments indicate  that  from  variable  Irish  rats  one  can  by  selection  obtain 
rats  transmitting  no  pattern  but  Irish.  The  theoretical  importance  of  this 
is  obvious.  Cross-breeding  and  selection  combined  are  means  by  which 
we  may  not  only  modify  existing  Mendelian  characters,  but  may  even 
create  new  ones.  They  are,  then,  factors  of  prime  importance  in  evolution, 
even  in  the  case  of  characters  which  vary  discontinuously. 


IN   RATS   AND  GUINEA-PIGS. 


MENDELIAN    INHERITANCE  OF   COAT-COLORS   AND  COAT-PATTERNS 

IN    RATS. 

While  pigment  character  and  color-pattern  are  both  inherited  in  Mendelian 
fashion,  the  two  are  entirely  independent  of  each  other.  They  are  separate 
and  uncorrelated  unit  characters.  Accordingly  we  find  that  each  type  of 
pigmented  coat-pattern,  namely,  self,  Irish,  and  hooded,  may  occur  either 
with  gray  or  with  black  pigmentation,  the  frequencies  with  which  they  occur 
in  the  respective  combinations  being  governed  by  the  laws  of  chance  and  of 
Mendelian  dominance.  This  will  appear  in  the  detailed  discussion  of  the 
experiments. 

Not  only  may  each  coat-pattern  occur  either  in  a  gray  or  in  a  black 
pigmented  individual,  but  it  may  occur  also  in  an  unpigmented  animal.  Para- 
doxical as  this  statement  may  seem,  it  is  capable  of  abundant  proof.  The 
coat-pattern  of  course  is  not  visible  in  an  unpigmented  (albino)  rat,  but  its 
presence  there  as  a  potentiality  can  be  demonstrated  as  certainly  as  the 
occurrence  of  a  recessive  character  in  a  heterozygous  dominant  individual. 
Nothing  but  the  presence  of  pigment  is  necessary  to  make  the  color-pattern 
manifest.  This  can  be  supplied  by  a  mating  with  a  pigmented  animal. 

Specific  pigment  potentialities  (gray,  black,  or  both)  are  likewise  present 
in  albinos.  Consequently  we  must  recognize  that  albinos  transmit  inactive 
both  pigments  and  color-patterns;  these,  however,  are  unseen  and  can  not 
be  made  visible  until  some  lacking  substance  borne  by  all  pigmented  indi- 
viduals is  supplied.  Characters  transmitted  in  this  inactive  state  have  been 
termed  by  one  of  us  (Castle,  :  05)  latent,  and  that  terminology  will  be  followed 
in  the  present  paper. 

ALBINISM    RECESSIVE    IN    RELATION    TO    AU,    TYPES    OF    PIGMENTATION. 

That  total  albinism  behaves  as  a  recessive  Mendelian  character  has  been 
recognized  independently  by  a  number  of  investigators,  among  the  earliest 
being  Correns  (:  01)  and  Cu^not  (:  02).  The  fact  has  been  abundantly  veri- 
fied in  the  case  of  mice  (see  Castle  and  Allen,  103),  rats  (Crampe,  '77-84; 
Bateson,  103;  Doncaster,  :o6),  rabbits  (Woods,  103;  Hurst  105;  and  Castle 
105),  and  guinea-pigs  (Castle,  105).  The  experiments  described  in  this 
paper  corroborate  those  of  Crampe  and  of  Doncaster  with  reference  to  rats. 
The  proportions  of  albinos  and  of  pigmented  individuals  in  mixed  litters 
are  close  to  the  Mendelian  expectations,  indicating  neither  selective  union 
of  gametes  nor  lessened  fertility  of  certain  sorts  of  unions.  Pigmented  rats, 
in  which  albinism  was  recessive,  when  mated  inter  se,  have  produced  129 
albinos  to  384  pigmented  young,  the  numbers  expected  being  128  and  385, 
respectively.  Albinos  mated  with  pigmented  individuals,  in  which  albinism 


8  INHERITANCE  OF  COAT-PIGMENTS  AND  COAT-PATTERNS 

was  recessive,  have  produced  201  albinos  to  244  pigmented  young,  or  45.1 
per  cent  albinos,  where  50  per  cent  are  expected.  The  deviation  of  nearly 
5  per  cent  from  expectation  in  the  latter  case  is  probably  a  matter  of  chance 
and  would  grow  less  with  more  extensive  observations,  for  in  studying  the 
inheritance  of  total  albinism  some  observers  record  an  excess  of  albinos 
(Castle,  105),  others  an  excess  of  pigmented  individuals  (Allen,  104),  while 
others  observe  very  close  agreement  with  expectation  (Cu^not,  Darbishire). 
This  is  as  we  should  expect  on  the  theory  of  probabilities  (see  Allen,  104,  p.  82). 

LATENT  PIGMENT  CHARACTERS  AND  COAT-PATTERNS. 

Latent  transmission  of  pigment  characters  through  albinos  is  a  matter 
requiring  fuller  consideration.  Assuming  for  the  time  being  its  correctness, 
and  knowing  the  well-established  facts,  (i)  that  gray  pigmentation  in  rats 
is  dominant  over  black  (Crampe,  Bateson,  Doncaster)  and  (2)  that  every 
sort  of  pigmentation  is  dominant  over  albinism,  we  reach  the  following 
conclusion.  On  Mendelian  principles  we  may  expect  partial  albinos  to  fall 
into  eighteen  actually  different  classes,  though  visibly  the  classes  number 
only  four,  namely,  gray  Irish,  black  Irish,  gray  hooded,  and  black  hooded. 
These  eighteen  classes  are  enumerated  in  the  four  central  columns  of  table  i , 
page  34.  Of  gray  Irish  individuals,  there  should  be  eight  actually  different 
classes  as  regards  gametic  output;  of  black  Irish  and  gray  hooded  indi- 
viduals, there  should  be  four  classes  each;  and  of  black  hooded,  two  classes. 

Albinos,  though  indistinguishable  in  appearance,  should  fall  into  nine 
different  classes  as  regards  the  latent  pigment  characters  and  color-patterns 
which  they  transmit.  The  nature  of  these  nine  classes  is  indicated  in  the 
last  column  of  table  i.  The  numerals  prefixed  to  the  class  designations  in 
table  i  indicate  the  frequencies  in  which  the  various  classes  may  be  expected 
to  occur  as  a  result  of  a  particular  sort  of  mating,  namely,  mating  inter  se 
gray  Irish  individuals  which  bear  recessive  the  three  characters,  total  albin- 
ism, black  pigmentation,  and  hooded  pattern,  individuals  designated 
GI(W.BH).  The  table  is  based  on  the  assumption  that  gray  Irish  indi- 
viduals of  the  sort  indicated  are  Mendelian  triple  heterozygotes,  the  three 
independent  pairs  of  allelomorphic  characters  being  pigmentation  versus 
albinism,  gray  versus  black,  and  Irish  versus  hooded.  In  support  of  the 
assumption  mentioned,  it  may  be  said  that  by  proper  breeding  tests  a  majority 
of  the  hypothetical  classes  have  been  shown  to  exist.  Thus,  of  the  eighteen 
hypothetical  pigmented  classes  enumerated  in  table  i,  all  except  four  have 
been  shown  to  occur,  those  four  being  GI(W),  GI(BH)*  BI*  and  BI(W)*. 
Further,  five  out  of  the  nine  supposed  classes  of  albinos  have  been  proved 

*  Demonstrated  to  exist  since  the  foregoing  was  written. 


IN  RATS  AND  GUINEA-PIGS.  9 

to  occur,  those  the  existence  of  which  has  not  yet  been  demonstrated  being 
W[BI],  W[GI]t  W[G-IH],  and  W[GB-I].  It  is  probable  that  more  extensive 
tests  would  demonstrate  the  existence  of  all  the  missing  classes,  as  no  special 
search  has  been  made  for  them,  the  demonstrations  obtained  being  for  the 
most  part  incidental  to  other  investigations. 

The  fundamental  assumption  on  which  table  i  rests  finds  further  justifi- 
cation in  the  numerical  proportions  in  which  the  various  classes  are  observed 
to  occur,  as  will  appear  from  an  examination  of  tables  2  and  3  (pp.  35,  36). 
In  these  tables  are  shown  the  theoretical  Mendelian  results  and  the  actual 
results  of  twenty- three  different  sorts  of  matings  involving  the  pigments 
and  color- patterns  which  have  been  discussed  in  the  previous  pages.  Table 
2  includes  only  matings  between  pigmented  animals;  table  3,  matings  be- 
tween a  pigmented  animal  and  an  albino.  Throughout  both  tables,  it  will 
be  seen,  the  expected  Mendelian  results  agree  quite  closely  with  those  actually 
observed.  Two  animals  possessing  the  same  recessive  character  have  in 
no  case  produced  offspring  bearing  the  corresponding  dominant  character. 
Further,  the  proportions  of  dominants  and  of  recessives  observed  in  mixed 
lots  of  young  agree  in  general  quite  closely  with  those  expected.  The  most 
striking  deviation  is  an  excess  of  black  hooded  young  (BH)  which  is  seen  in 
the  totals  for  both  table  2  and  table  3.  This,  however,  is  not  large  enough 
and  does  not  occur  with  enough  uniformity  to  warrant  one  in  regarding  it 
as  other  than  accidental.  The  totals  for  tables  2  and  3  combined  are: 

GI  GH  BI  BH  W         Total. 

Observed 69         132  130        186         168        685 

Expected 56         137.5         136         158         187 

In  order  of  size  the  groups  are  as  expected,  except  the  last  two,  in  case  of 
which  the  order  is  reversed. 

In  addition  to  the  experiments  recorded  in  tables  2  and  3,  the  following 
observations  may  be  mentioned  as  corroborative  evidence  of  the  Mendelian 
behavior  of  pigment  characters,  color-patterns,  and  albinism  in  rats. 

Black  hooded  individuals  bearing  white  recessive,  mated  inter  se,  give 
two  classes  of  offspring,  albinos  and  black  hooded.  These  albinos  have  been 
shown  to  bear  without  exception  the  black  hooded  pattern  in  a  latent  con- 
dition. Of  the  black  hooded  offspring,  there  should  be  two  classes — pure 
hooded  (free  from  albinism)  and  hooded  individuals  bearing  albinism  re- 
cessive, the  latter  being  twice  as  numerous  as  the  former.  Out  of  22  off- 
spring tested,  14  had  albinism  recessive  and  8  were  pure  hooded  individuals. 

Matings  of  pure  black  hooded  individuals  with  black  hooded  ones  bearing 
white  recessive  gave  all  black  hooded  offspring,  109  in  number.  One-half 
of  these  should  be  pure  hooded,  and  of  the  limited  number  tested  this  was 
found  to  be  true. 


10 


INHERITANCE  OF   COAT-PIGMENTS  AND  COAT-PATTERNS 


The  evidence  contained  in  tables  2  and  3  (pp.  35,  36),  is  presented  in  the  form 
of  summaries  only.  To  give  an  idea  of  the  sources  from  which  it  was  obtained, 
a  genealogy  may  be  considered  in  detail  (fig.  i). 


Wild.  oray 


Wild  white 


J12,W[BB] 


4  Ai,Gl(- 


£  34,  G-lfw-BIf) 


£  26,  Gl(w-Bff) 


£«* 

GI(W-B) 

ventral  white  patch,  below  the  average  size 

o  142 
f  143 
£144 
J  145 
<f  146 
4  147 

GI(H) 
GI(W-BM) 
G 
BI(W-H) 
GHfB) 
BH 

large  "white  "ventraX  patch 
gray  self 
large  "white  ventral  patch 

FIG.  l. 

A  wild  gray  female  and  a  wild  white  male  were  trapped  together.     Nothing 
is  known  of  their  ancestry,  but  they  were  probably  derived  from  escaped 
tame  albinos  which  had  mated  with  wild  gray  rats.     This  pair  of  captured 
rats  produced  gray  Irish  young, two  of  which  (Ai  and  ,42),  when  mated  with 
ordinary  albinos,  produced  gray  Irish  young  which  bore,  as  recessive  char- 
acters, total  albinism,  black  pigmentation,  and  the  hooded  pattern.     Two 
of  these  triple  heterozygotes   ($34  and  c?i6)   were  now  mated  together 
and  produced  a  litter  of  seven  young  (141-147,  fig.  i),  which  were  tested 
as  to  their  gametic  condition,  with  the  results  indicated  in  fig.  i.     Five  of 
the  7  were  gray  pigmented  and  2  black,  but  no  two  were  alike  in  gametic 
character.     Males  146  and  147  were  mated  with  black  hooded  females  and 
with  albino  females  of  black  hooded  parentage,  producing  nothing  but  hooded 
young.     This  result  showed  that  they  bore  no  coat-pattern  except  the  hooded 
one,  and  were  free  from  recessive  albinism. 

The  young  of  6^147  were  all  black  pigmented,  showing  that  animal  to 
be  entirely  homozygous,  but  some  of  the  young  of  6^146  were  gray  pig- 
mented, some  black  pigmented,  showing  that  he  bore  black  pigmentation 
as  a  recessive  character,  being  otherwise  homozygous. 


IN  RATS   AND   GUINEA-PIGS.  II 

The  gray  Irish  female  (141)  has  produced,  by  hooded  males,  young  of  the 
Irish  pattern  only,  19  in  number,  some  of  them  being  gray,  and  others  black 
pigmented;  she  has  also  produced  albino  young  by  hooded  males  bearing 
recessive  albinism.  She  accordingly  bears  black  pigmentation  and  total 
albinism  as  recessive  characters,  but  is  otherwise  homozygous,  for  if  she  bore 
the  hooded  pattern,  half  her  pigmented  young  should  have  been  hooded. 
By  similar  tests,  the  gametic  formulae  of  individuals  142  to  145  were  estab- 
lished, as  given  in  fig.  i. 

Experiments  such  as  these  show  the  complete  mutual  independence  of 
pigmentation,  color-pattern,  and  total  albinism,  as  has  been  repeatedly 
stated.  They  indicate,  further,  that  partial  albino  rats  (hooded  and  Irish) 
regularly  form  gametes  which  bear  the  partial  albino  condition,  either  in 
part  or  in  all  of  their  gametes;  and  that  the  extent  of  the  white  areas  on 
partial  albinos  varies  in  continuous  fashion,  a  self  condition  being  some- 
times obtained  as  an  extreme  variation  of  Irish.  This  last  point  is  impor- 
tant as  showing  that  no  complete  discontinuity  exists  between  self  and 
Irish  patterns  in  rats,  since  the  gap  ordinarily  existing  between  them  can 
be  bridged  with  a  complete  series  of  intermediate  conditions.  The  same  is 
true  of  the  Irish  and  hooded  conditions.  Though  ordinarily  discontinuous 
and  behaving  as  alternative  Mendelian  characters  in  inheritance,  we  can  by 
cross-breeding  and  selection  bridge  the  gap  between  them. 

MODIFICATION  OF   HOODED  PATTERN  BY  CROSSING   WITH  IRISH. 

Hooded  and  Irish  individuals  differ,  as  already  stated,  simply  in  the 
extent  of  their  pigmentation.  In  hooded  individuals,  the  pigmentation  is 
restricted  to  the  hood  (which  covers  the  head  and  shoulders)  and  to  the 
dorsal  stripe;  in  Irish  individuals,  the  entire  coat  is  pigmented  except  a 
ventral  patch  of  variable  size.  Extension  of  the  dorsal  stripe  of  the  hooded 
pattern,  so  as  to  cover  the  entire  dorsal  surface  and  the  sides  of  the  body, 
would  yield  the  Irish  pattern.  It  was  our  purpose  in  one  set  of  experiments 
to  see  whether  such  modification  could  be  brought  about,  and  to  see  also 
how  far  an  opposite  modification  of  the  hooded  pattern  (further  reduction 
of  the  pigmented  areas)  could  be  effected.  Before  considering  those  experi- 
ments in  detail,  we  may  inquire  what  effect  upon  the  hooded  pattern  a  cross 
with  the  Irish  pattern  will  have. 

The  hooded  stock  used  in  this  investigation  consisted  of  individuals  which 
bore  albinism  recessive,  but  had  no  Irish  ancestors,  so  far  a?  known.  The 
extent  of  the  dorsal  stripe  varied  considerably,  so  that  it  was  found  desir- 
able to  measure  it  as  accurately  as  possible  in  each  individual.  This  was 
done,  first,  by  estimating  the  width  of  the  stripe  in  relation  to  the  total 
width  of  the  back;  next  was  estimated  the  extent  to  which  the  stripe  was 


12  INHERITANCE  OF   COAT-PIGMENTS  AND  COAT-PATTERNS 

interrupted,  that  is,  the  proportion  of  the  total  stripe  length  which  was 
pigmented.  The  product  of  these  two  ratios,  relative  width  and  relative 
length  of  the  back-stripe,  when  multiplied  by  100,  gives  an  approximation  to 
the  per  cent  of  dorsal  surface  (posterior  to  the  hood)  which  was  pigmented. 
Such  a  valuation,  or  grade,  was  obtained  for  each  hooded  individual. 

The  grades  for  183  individuals  (lot  A,  fig.  2),  belonging  to  three  successive 
generations,  yield  the  variation  curve  shown  in  fig.  3,  A .  In  obtaining  this 
curve  grades  from  o  to  9  inclusive  were  grouped  in  a  single  class,  the  mean 
value  of  which  is  4.5;  grades  from  10  to  19  inclusive,  in  a  class  the  mean 
value  of  which  is  14.5,  and  so  on.  The  ordinates  in  fig.  3,  A,  show  the  fre- 
quencies of  these  classes.  The  grades  fall  into  four  classes,  the  one  with 
greatest  frequency  (modal  class)  being  the  one  with  the  lowest  mean  (4.5), 

the  frequencies  of  the  other  classes 
decreasing   in    order  upward.      The 
average  grade  for  the  entire  183  in- 
1       dividuals   is   13.3   (see  vertical   line 

r-J        in  fig.  3,  A). 

The  hooded  stock  was  now  crossed 
with  an  Irish  stock,  and  the  Irish 
cross-breeds  thus  produced  were  in 
j  turn  crossed  back  with  the  parental 

t  hooded  stock.     From  this  last  cross 

'  ,         |  ' '    were  obtained  both  Irish  and  hooded 

tj.-~-(uvi>jj'/-r4.j.  H  (JLtot  C )    I       ff        .  ,  ,  ,.  f 

i  offspring,    in    proportions    sensibly 

H(LotE)  equal.     These  hooded  offspring  form 

a  group  (lot  B,  fig.  2)  derived  through 
H  (Lot  F)  one  parent  directly  from  the  original 

FIG.  2.- Ancestry  of  lots  A  to  F  of  hooded  rats.       hooded    Stock    (lot   A),    but    through 
Read  downward.  the  other  parent  from  the  Ifish  cross. 

breeds.  One  of  the  two  gametes,  accordingly,  entering  into  each  zygote  had 
a  chance  to  be  modified  by  association  with  the  Irish  character.  Lot  B 
includes  126  individuals,  with  back-stripes  on  the  average  nearly  twice  as 
extensive  as  those  of  lot  A,  the  average  grades  of  the  two  lots  being  13.3 
for  lot  A,  and  21.2  for  lot  B.  The  form  of  the  variation  curve  for  lot  B  is 
shown  in  fig.  3,  B.  The  modal  class  is  higher  than  in  lot  A,  and  the  range 
of  variation  is  extended  upward  so  as  to  include  seven  classes,  the  highest 
one  having  a  mean  grade  of  64.5.  Accordingly,  we  conclude  that  a  cross 
with  the  Irish  stock  raises  considerably  the  average  size  of  the  dorsal  stripe 
in  hooded  rats,  as  well  as  the  range  of  variation  upward  in  size  of  stripe. 
In  the  same  litters  with  the  hooded  rats  forming  lot  B,  were  born  rats  of 
two  other  sorts,  Irish  and  albino  (fig.  2) .  The  former  bred  inter  se  produced 


IN   RATS   AND  GUINEA-PIGS. 


Individuals 


KQ 

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34.5  4-4.5  54.5  6*5  74.5 


FIG.  3.— Variation  in  extent  of  the  back-stripe  in  lots  A  to  F  of  hooded  rats,  showing  the  effects 
breeding  with  rats  of  Irish  pattern.     The  positions  of  the  means  are  shown  by  vertical  lines 


of  cross- 


14  INHERITANCE  OF  COAT-PIGMENTS  AND  COAT-PATTERNS 

the  hooded  rats  forming  lot  C  (fig.  2) ;  the  albinos,  when  back-crossed  with 
the  original  hooded  stock,  produced  hooded  offspring,  which  are  included 
in  lot  D  (fig.  2),  as  are  also  the  hooded  descendants  of  lot  B  bred  inter  se. 

Lot  D,  accordingly,  is  a  group  of  individuals  of  ancestry  more  mixed  than 
lot  B,  but  on  the  whole  similar.  It  shows  a  similar  variability  (fig.  3,  D), 
but  is  of  a  higher  average  grade  and  shows  a  tendency  to  form  a  second 
mode,  a  group  of  wide-striped  individuals,  with  a  modal  grade  of  54.5,  i-e., 
individuals  in  which  the  stripe  is  about  half  as  wide  as  the  entire  dorsal 
surface  (compare  pi.  i,  fig.  2). 

Lot  C  (fig.  2  and  fig.  3,  C)  contained  only  36  individuals.  Its  average 
grade  is  almost  identical  with  that  of  lot  D,  23.5  as  compared  with  23.9. 
Its  range  also  is  similar;  but  the  curve  itself  is  flatter,  yet  with  the  same 
tendency  to  become  subdivided  into  two  groups,  one  with  a  wider  stripe, 
the  other  with  a  stripe  like  that  of  the  original  hooded  stock  (lot  .4).  Two 
other  lots  of  hooded  rats  (E  and  F,  figs.  2  and  3)  belonging  to  this  series 
have  also  been  studied.  Lot  E  consisted  of  the  hooded  young  of  certain 
individuals  of  lot  D,  those  having  the  lesser  amount  of  Irish  ancestry,  while 
lot  F  was  derived  from  certain  individuals  of  lot  E  bred  inter  se.  The  parents 
of  lot  E  had  an  average  grade  of  21.8,  similar  to  that  of  lot  B  (21.2),  or  to 
the  lower  modal  group  of  lot  D;  the  average  grade  of  lot  E  itself  was  very 
similar,  namely  21.1.  The  parents  of  lot  F  had  an  average  grade  of  17.8; 
lot  F  itself  had  an  average  grade  of  12.9.  The  variation  curves  for  lots  E 
and  F  are  unimodal,  like  those  for  the  original  lot  A,  giving  no  evidence  of 
a  tendency  to  form  a  wide-striped  variety. 

MODIFICATION    OF    HOODED    PATTERN    BY    SELECTION. 

The  close  agreement  between  the  grades  of  the  parents  and  children  in 
lot  F,  and  measurably  also  in  lot  F,  suggests  that  one  can  at  will  either  in- 
crease or  decrease  the  width  of  the  stripe  in  a  stock  of  hooded  rats.  To 
test  this  matter  more  fully,  selection  has  been  made  both  for  reduced  and  for 
increased  size  of  stripe. 

SELECTION   FOR  REDUCED  STRIPE. 

Eleven  individuals  of  lot  A,  having  narrow  or  interrupted  dorsal  stripes 
(average  grade  n),  formed  the  beginning  of  a  selection  experiment  for 
reduced  pigmentation  (compare  pi.  i,  fig.  4).  They  produced  83  young, 
lot  G,*  figure  4,  with  an  average  grade  of  9.6,  and  a  form  of  variation 
curve  suggesting  the  presence  of  two  groups  of  individuals,  each  with  a  dorsal 

*On  account  of  the  smaller  range  of  variation  in  this  series  of  experiments,  the  size  of 
the  classes  was  made  only  half  as  great  as  in  the  case  of  lots  A-F,  fig.  3.  The  mean  values 
of  the  classes  in  lots  G-I  are  2,  7,  12,  17,  etc. 


IN   RATS   AND  GUINEA-PIGS- 


\ 


70 


60 


50 


\ 


4-0 


stripe  of  different  extent,  one  group  being  of  moderate  size  (12),  the 
other  extremely  reduced  (2).  Twenty-eight  individuals  whose  grades  fell 
in  the  lower  part  of  curve  G  (fig.  4),  and  averaged  7.8,  were  the  parents  of 
the  next  generation,  group  77.  Individuals 
This  generation  consisted  of 
6 1  individuals  having  an  aver- 
age grade  of  5.6.  The  varia- 
tion curve  for  this  group  (77, 
fig.  4),  shows  a  nearly  com- 
plete dropping  out  of  individ- 
uals with  the  stripe  of  moder- 
ate size,  such  as  occurred  in 
group  G.  All  the  classes 
above  7  are  small  and  the 
upward  range  stops  at  22.  A 
few  individuals  of  lot  77  had 
no  back-stripe  at  all  (com- 
pare pi.  i,  fig.  3).  These  with 
some  individuals  having  a 
much  reduced  stripe  were  se- 
lected as  parents  for  the  next 
generation,  lot  7.  The  aver- 
age grade  of  the  parents  was 
in  this  case  1.7;  that  of  their 
34  young  (lot  7,  fig.  4)  was  4. 
Twenty-four  of  the  34  young 
fell  in  the  lowest  class,  mean 
grade  2,  while  none  were 
above  class  12. 

The  effect  of  selection  in 
this  series  of  experiments  is 
clear  from  a  comparison  of 
the  four  curves,  A,  G,  77, 
and  7,  fig.  4.  Selection  has 
steadily  lowered  the  average 
amount  of  pigmentation  in  the^r<?c/e 

race   by  reducing  the   Upward    I?IG-  4-~Effect  of  selection  for  back-stripe  of  reduced  size  in 
£   ^  •    ±-  hooded  rats,  lots  A,G,H,  and  /. 

range  of  the  variation  curve, 

thus  eliminating  the  more  abmodal  variations,  reducing  the  skewness  of  the 
curve,  and  diminishing  the  distance  between  mean  and  mode.  One  of  the 
noteworthy  features  of  the  case  is  the  absence  of  what  may  properly  be 


17      32      87      33      37     +2 


i6 


INHERITANCE  OF  COAT-PIGMENTS  AND  COAT-PATTERNS 


called  regression.  The  filial  average  does  not,  with  any  uniformity,  lag 
behind  the  parental  average,  in  the  process  of  displacement  downward  in  the 
variation  figure.  This  fact,  together  with  the  decreasing  skewness  of  the 
variation  curve,  indicates  that  the  effects  of  selection  in  reducing  the  extent 

Individuals 


70 


6O 


\ 


V 

\ 


30 


2O 


IO 


GraCf€4.5  I4.S  24*5  34.5  44.5  5-4.5  64.5  74.5  64.5 

FIG.  5— Effect  of  selection  for  back-stripe  of  increased  size  in  hooded  rats,  lots  A,  J,  K,  L,  and  M. 

of  the  pigmentation  will  be  permanent,  that  is,  that  a  stable,  narrow- 
striped  variety  of  hooded  rats  can  be  established  by  selection,  and  that 
this  variety  will  breed  true- 


IN  RATS  ANt>  GUINEA-PIG§.  t^ 

SELECTION  FOR  STRIPS  OF  INCREASED  SIZE. 

Selection  in  an  opposite  direction,  for  a  wider  and  more  continuous  stripe, 
leads  to  similar  conclusions.  In  this  series  of  experiments,  the  wide-striped 
parents  used  were  obtained  as  a  result  of  crosses  between  hooded  and  Irish 
individuals.  Such  crosses  tended  to  widen  the  stripe  of  the  hooded  offspring, 
the  widening  effect  being  permanent,  as  the  sequel  shows. 

Hooded  individuals,  either  parents  of  wide-striped  young,  or  such  as 
themselves  had  wide  stripes,  were  selected  from  lots  B  and  C,  and  mated 
together.  The  average  grade  of  the  parents  was  12.  Their  young  (lot  /, 
fig.  5),  in  in  number,  had  a  higher  average  grade  than  the  parents,  namely, 
15.8.  The  curve  was  similar  in  form  to  the  curve  (4)  produced  by  the 
original  hooded  stock. 

A  second  group  of  wide-striped  individuals,  of  higher  grade  than  that 
just  described,  though  of  similar  ancestry,  produced  a  group  of  70  young 
(lot  K,  fig.  5),  like  themselves  in  width  of  stripe.  The  average  grade  of 
the  parents  was  23,  that  of  the  young  (lot  K)  was  22.2.  The  variation 
curve  (K,  fig.  5)  is  a  very  flat  one,  similar  to  curve  B,  fig.  3. 

A  third  group  of  wide-striped  parents,  of  still  higher  grade,  was  taken 
chiefly  from  lot  D.  Their  average  grade  was  40.7.  They  produced  64 
young  (lot  L,  fig.  5),  having  an  average  grade  of  36.2.  The  form  of  variation 
curve  given  by  lot  L  is  similar  to  that  given  by  lot  D,  flat  and  bimodal. 
The  upper  mode,  however,  is  relatively  taller,  indicating  that  the  upper 
(or  wide-striped)  group  is  increasing  at  the  expense  of  the  lower,  as  a  result 
of  the  selection,  and  that  the  original  two-peaked  condition  of  curve  D  was 
due,  not  to  the  heterogeneity  of  the  material  included  in  lot  D,  but  to  the 
fact  that  part  of  the  gametes  formed  by  the  cross-bred  individuals  trans- 
mitted a  modified  (wide-striped)  condition.  This  wide-striped  condition 
was  now  (in  lot  L)  in  process  of  segregation  through  the  action  of  selection, 
and  in  a  fair  way  to  form  a  stable  wide-striped  race.  Indeed,  in  the  best 
wide-striped  pairs  of  lot  L  this  seems  already  to  have  been  accomplished; 
for  certain  extremely  wide-striped  individuals  (partly  of  lot  L  and  partly 
of  a  later  generation),  which  had  an  average  grade  of  50,  produced  the  19 
young  included  in  lot  M,  fig.  5,  which  had  an  average  grade  of  53.  The 
mode  for  this  curve  is  close  to  the  mean,  being  54.5,  and  the  curve  itself 
is  nearly  symmetrical,  indicating  approach  to  a  condition  of  stability. 

The  theoretical  importance  of  the  results  of  these  selection  experiments 
is  evident.  Characters  can  be  permanently  modified  by  selection,  contrary 
to  the  view  of  De  Vries  that  such  modification  is  impossible. 

The  objection  may  be  raised  that  the  permanency  of  the  modification  has 
not  been  fully  established,  for  later  generations  might  show  regression  to 
a  narrower-striped  condition.  While  admitting  that  this  objection  has 


1 8  INHERITANCE   OF  COAT-PIGMENTS  AND  COAT-PATTERNS 

some  force,  the  fact  may  be  pointed  out  that  regression  does  not  occur  in 
the  experiments  made,  and  if  regression  were  to  occur  at  all,  we  should  expect 
it  to  occur  and  to  be  greatest  in  the  early  stages  of  a  selection  experiment. 
The  average  grade  of  the  offspring  is  sometimes  greater,  sometimes  less,  than 
that  of  their  parents,  as  we  should  expect  if  these  deviations  are  purely  the 
result  of  chance.  Table  4  (p.  36)  shows  the  nature  and  amount  of  the 
deviation  following  each  selection  made.  No  great  importance  attaches  to 
the  exact  amounts  of  deviation  indicated  in  this  table.  For  in  computing 
the  parental  averages  all  parents  were  weighted  alike,  but  a  better  method 
of  procedure  would  have  been  to  weight  each  parent  in  proportion  to  the 
number  of  young  which  it  produced.  The  general  nature  of  the  result 
obtained  is,  however,  not  vitiated  by  the  procedure  followed;  it  serves  to 
show  the  absence  of  any  uniform  regression  of  either  a  plus  or  a  minus  nature. 

INDIVIDUAL   SPOTS    OF    GUINEA-PIGS  NOT   UNIT    CHARACTERS. 

In  an  earlier  paper  (Castle,  :  05)  the  fact  was  pointed  out  by  one  of 
us  that  in  partial-albino  guinea-pigs,  as  in  other  mammals,  the  pigmented 
areas  are  not  distributed  without  order  over  the  body,  but  occur  in  regions 
fairly  definite.  These  regions  in  the  guinea-pig  consist  of  live  paired  "pig- 
ment centers,"  dorso-lateral  in  position.  To  define  their  location  more 
accurately,  they  have  been  called  the  eye,  ear,  shoulder,  side,  and  rump  areas. 

The  hypothesis  was  advanced  (Castle,  :  05,  p.  45)  that  these  several  areas 
might  represent  separately  heritable  unit  characters,  although  experience 
had  clearly  shown  that,  if  such  were  the  case,  they  were  not  inherited  in 
ordinary  Mendelian  fashion.  To  test  this  hypothesis  more  fully,  we  have 
attempted  to  establish  races  of  guinea-pigs  which  should  possess  certain  of 
the  pigmented  areas,  but  should  lack  others.  For  this  purpose  experiments 
have  been  made  with  three  different  patterns  of  partial  albinism,  arbitrarily 
chosen  from  those  which  occur  among  ordinary  spotted  guinea-pigs.  These 
are  the  Dutch-marked  pattern  (pi.  2,  fig.  3),  the  head-spot  pattern  (pi.  2,  fig.  2) 
and  the  nose-spot  pattern  (pi.  2,  fig.  i). 

COLOR-PATTERNS  SELECTED. 

In  the  Dutch -marked  pattern,  all  the  typical  spots  are  present  except  the 
shoulder  spots.  The  absence  of  shoulder  spots  produces  a  white  belt  or 
girdle  round  the  body.  Anterior  to  the  girdle,  eye  and  ear  spots  are  present 
and  usually  confluent  on  the  same  side  of  the  head,  but  not  across  the  median 
plane.  Posterior  to  the  girdle,  the  large  side  spots  and  the  small  rump  spots 
are  present  and  confluent  over  the  back  as  well  as  lengthwise  of  the  body, 
forming  a  large  blanket  of  pigment  covering  the  whole  posterior  part  of  the 
body,  but  not  extending  out  upon  the  feet.  This  is  the  ideal  "Dutch- 
marked"  type  of  the  fancier.  Occasionally  it  is  fully  realized,  but  more 
often  minor  departures  from  the  ideal  occur,  such  as  asymmetry  of  the 


PLATE  2 


COLOR-PATTERNS    SELECTED    IN    EXPERIMENTS    WITH     GUINEA-PICS. 

Fig.  I.  Nose-spot  pattern  (Series  N). 
Fig.  2.  Head-spot  pattern  (Series  H). 
Fig.  3.  Dutch-marked  pattern  (Series  D). 

The  rough  (resetted)  condition  of  the  coat  shown  in  Figs.  I  and  2  is  a  character 
wholly  independent  of  color-pattern. 


IN  RATS  AND  GUINEA-PIGS.  ic) 

pigmentation  of  the  two  sides  of  the  body,  failure  of  the  eye  and  ear  spots 
to  coalesce,  or  entire  absence  of  one  of  the  two,  absence  of  one  or  both  side 
spots,  or  the  occurrence  of  a  white  gap  between  side  and  rump  spots.  Our 
attempts  to  fix  this  pattern  by  selection,  continued  through  a  series  of  gen- 
erations, have  not  been  successful. 

Animals  of  the  head-spot  pattern  (pi.  2,  fig.  2)  differ  from  Dutch-marked 
chiefly  in  the  entire  absence  of  pigmentation  posterior  to  the  girdle,  though 
the  pigmentation  on  the  side  of  the  head  also  is  usually  less  extensive  than 
in  Dutch-marked  animals.  Because  of  the  practical  difficulty  of  distin- 
guishing eye  spots  from  ear  spots,  when  the  two  are  confluent  and  similarly 
pigmented,  we  have  throughout  this  series  of  experiments  treated  the  two 
as  forming  a  single  pair,  which  we  call  head  spots. 

When  the  head  pigmentation  is  most  reduced,  only  the  retina  of  the  eye 
is  pigmented.  Then  the  head-spot  type  passes  over  into  the  black-eyed 
white  type.  In  tabulating  our  results,  however,  we  have  treated  the  two 
types  as  distinct.  The  head-spot  pattern,  like  the  Dutch-marked,  we  have 
been  unable  to  "fix." 

The  nose-spot  pattern  occurs  less  frequently  than  the  two  already  de- 
scribed. The  entire  body  is  white,  except  that  part  of  the  head  which  is 
situated  anterior  to  the  eyes.  This  part  of  the  body  is  usually  unpigmented 
in  spotted  guinea-pigs,  which  fact  makes  the  occasional  occurrence  of  the 
nose-spot  pattern  more  striking.  Our  attempts  to  fix  this  pattern  have 
succeeded  scarcely  better  than  those  with  the  Dutch-marked  and  head-spot 
patterns. 

In  addition  to  our  attempts  to  fix  particular  color-patterns  arbitrarily 
chosen,  we  have  made  records  of  the  pigment  distribution  on  several  hun- 
dred spotted  guinea-pigs  of  known  ancestry,  with  a  view  to  ascertaining 
whether  or  not  particular  spots  are  inherited.  The  question  for  which  an 
answer  was  sought  may  be  stated  thus:  Does  an  animal  having,  let  us  say,  an 
eye  spot,  produce  young  with  eye  spots  any  oftener  than  animals  bearing  a 
like  amount  of  pigmentation  but  without  eye  spots  ?  If  this  question  receives 
an  affirmative  answer,  then  we  may  conclude  that  eye  spots,  as  such,  are 
inherited,  and  it  will  remain  to  ascertain  the  law  governing  their  inheritance. 

The  evidence  bearing  on  this  question  can  best  be  presented  in  the  form 
of  tables,  showing  what  areas  of  the  young  are  pigmented  in  comparison 
with  those  of  their  parents.  Such  a  table  has  been  constructed  for  each  of 
the  successive  generations  in  which  selection  was  exercised  for  the  fixation 
of  a  particular  coat-pattern.  By  a  comparison  of  the  tables  for  the  succes- 
sive generations,  we  may  learn  whether  selection  was  having  any  effect  in 
fixing  the  desired  coat-pattern.  For,  if  it  was  doing  so,  later  generations 
should  show  closer  agreement  than  earlier  ones  between  parents  and  children 
as  regards  distribution  of  pigment  on  the  body. 


20  INHERITANCE   OF  COAT-PIGMENTS   AND  COAT-PATTERNS 

DUTCH-MARKED  SERIES    (SERIES  p). 

The  data  for  inheritance  of  spots  in  the  Dutch-marked  series  are  con- 
tained in  tables  5  to  14.  The  individuals  are  arranged  in  a  series  of  groups, 
each  group  having  a  larger  average  amount  of  Dutch-marked  ancestry  than 
the  foregoing  one.  For  convenience  in  description,  we  may  use  the  symbol 
D  to  designate  any  Dutch-marked  animal,  followed  by  a  numeral  to  indicate 
the  amount  in  generations  of  its  Dutch-marked  ancestry.  On  this  basis  an 
original  Dutch-marked  animal,  one  without  Dutch-marked  ancestors,  would 
be  designated  DQ;  a  Dutch-marked  animal  with  one  generation  of  Dutch- 
marked  ancestry,  D\ ;  one  with  two  generations  of  Dutch-marked  ancestry, 
D2 ;  and  one  with  three  generations  of  Dutch-marked  ancestry,  D3 .  Simi- 
larly a  Dutch-marked  animal,  one  parent  of  which  was  Dutch-marked,  but 
the  other  not,  might  be  designated  D\ ;  the  Dutch-marked  young  of  D\ 
individuals  might  be  called  D\\  ;  etc. 

The  young  of  our  Dutch-marked  animals  have  been  divided  into  four 
groups,  the  average  amounts  of  Dutch-marked  ancestry  in  these  groups 
being  respectively  ^,  i,  2,  and  3  generations.  Groups  of  individuals  were 
at  first  created  which  came  midway  between  the  groups  mentioned,  but 
these  differed  so  little  from  the  adjacent  groups  that  we  decided  finally  to 
abolish  them,  dividing  them  equally  between  the  groups  to  which  they 
were  intermediate.  Thus  a  group,  DI%,  was  divided  equally  between  the 
groups  DI  and  D2,  and  a  group,  D2\,  between  the  groups  D2  and  D3. 

For  each  individual  observed,  a  record  was  made  of  what  spots  it  possessed 
and  what  spots  its  parents  possessed.  These  records  are  shown  in  tables 
5,  7,  9,  and  1 1  for  the  four  groups  of  individuals  having  the  mean  ancestral 
values  D$,  DI,  D2,  and  Z>3  respectively.  Bach  table  contains  separate  records 
for  head,  shoulder,  side,  and  rump  spots.  The  significance  of  the  various 
entries  will  be  sufficiently  clear  after  an  examination  of  a  single  category 
(head  spots)  in  table  5.  There  are  for  the  individuals  examined  three  pos- 
sible conditions  as  regards  head  spots.  An  individual  may  have  two  such 
spots  (one  on  either  side  of  the  head) ;  it  may  have  only  one  head  spot 
(right  or  left,  as  the  case  may  be) ;  or  it  may  have  no  head  spots.  Records 
are  made  for  these  three  conditions  separately  in  the  horizontal  columns 
marked  2  spots,  i  spot,  and  o  spots  respectively.  For  the  possible  head- 
spot  conditions  of  the  two  parents  considered  jointly,  five  categories  are 
necessary,  4,  3,  2,  i,  and  o.  These  are  indicated  in  vertical  columns  in  the 
tables.  An  entry  in  the  upper  left-hand  square  of  the  table  signifies  an 
individual  having  a  pair  of  head  spots,  born  of  parents  both  of  which  were 
similarly  marked;  an  entry  in  the  next  square  to  the  right  means  an  indi- 
vidual having  likewise  a  pair  of  head  spots,  but  born  of  parents  only  one  of 
which  had  a  pair  of  head  spots,  the  other  having  one  head  spot  only.  The 


IN  RATS  AND  GUINEA-PIGS.  21 

table,  accordingly,  it  will  be  observed,  represents  the  summation  of  a  large 
amount  of  detailed  information  concerning  the  distribution  of  spots  in  off- 
spring and  parents,  respectively. 

For  convenience  in  comparing  the  categories  of  individuals  one  with 
another,  the  entries  in  the  tables  are  expressed  in  per  cents,  as  well  as  in 
numbers  of  cases  observed.  Thus  in  table  5,  left-hand  vertical  column, 
entries  are  made  concerning  the  head-spot  conditions  of  49  young  born  of 
parents  both  of  which  had  two  head  spots.  Of  these  49  young,  42,  or  85.7 
per  cent,  had  two  head  spots,  as  each  of  the  parents  had;  6,  or  12.3  per  cent, 
had  only  one  head  spot;  while  i  individual,  or  2  per  cent,  had  no  head  spots. 
The  other  columns  of  the  table  are  to  be  interpreted  in  a  similar  way. 

Glancing  over  the  four  divisions  of  table  5,  which  includes  observations  on 
1 1 7  young,  we  notice  that  the  offspring  of  parents  both  of  which  possess  a 
particular  pair  of  spots  (left-hand  vertical  column)  possess  that  same  pair 
of  spots  in  from  70  to  86  per  cent  of  all  cases.  The  agreement  between 
parents  and  offspring  is  greatest  (85.7  per  cent)  in  the  case  of  head  spots, 
and  least  (70  per  cent)  in  the  case  of  side  spots. 

From  the  last  vertical  column  of  table  5  we  learn  that  when  both  parents 
lack  shoulder  or  side  spots,  50  to  58  per  cent  of  their  young  lack  the  corre- 
sponding pair  of  spots  entirely. 

These  facts  seem  to  indicate  a  rather  strong  degree  of  inheritance  of  par- 
ticular spots  upon  the  body  of  the  guinea-pig;  so  we  at  first  interpreted 
them,  but  more  extended  observations  limit  this  conclusion  considerably. 

The  four  divisions  of  table  5  are  combined  in  table  6,  which  accordingly 
shows,  for  the  body  as  a  whole,  to  what  extent  particular  spots  are  inherited 
in  the  group  of  117  individuals  with  which  tables  5  and  6  deal.  Inheritance 
of  paired  spots  is  seen  to  be,  on  the  average,  measured  by  78.8  per  cent  (left- 
hand  upper  square,  table  6) ;  absence  of  paired  spots  is  seen  to  be  measured 
in  inheritance  by  55  percent  (lower  right-hand  square,  table  6).  Further, 
the  fewer  spots  the  parents  have,  the  fewer  their  young  have,  as  a  glance  at 
the  horizontal  columns  of  table  6  will  show.  This  is  true  not  only  of  the 
body  as  a  whole  (table  6),  but  also  of  its  individual  parts  (table  5).  Par- 
ticular amounts  of  pigmentation  are  accordingly  inherited  quite  strongly; 
distribution  of  that  pigment  in  certain  of  the  typically  pigmented  areas, 
rather  than  in  others,  is  really  not  inherited,  as  we  shall  see,  though  such 
is  the  impression  first  given  by  these  tables. 

For  the  second  group  of  individuals  of  Dutch-marked  ancestry  (mean 
ancestral  condition,  DI),  the  observations  concerning  spot  inheritance  are 
given  in  tables  7  and  8;  for  the  third  group  (D2),  in  tables  9  and  10;  and  for 
the  fourth  group  (#3),  in  tables  n  and  12.  The  numbers  of  individuals  in 
these  several  groups  are  30,  178,  and  136,  respectively,  making  in  the  Dutch- 
marked  series  as  a  whole  461  individuals  observed. 


22  INHERIT ANCE  OF  COAT-PIGMENTS  AND  COAT-PATTERNS 

To  obtain  averages  for  the  entire  series,  with  which  the  values  for  the 
several  groups  might  be  compared,  tables  5, 7,  9,  and  1 1  have  been  combined 
to  form  table  13,  which  accordingly  shows  the  average  degree  of  inheritance, 
in  the  entire  series,  of  head  spots,  shoulder  spots,  side  spots,  and  rump 
spots,  respectively.  Similarly,  tables  6, 8,  io,and  12  have  here  been  combined 
in  table  14,  which  accordingly  shows  in  general  to  what  extent  paired  spots 
are  inherited  in  the  Dutch-marked  series. 

A  comparison  of  the  inheritance  coefficients  (per  cents)  in  corresponding 
places  of  tables  5,  7,  9,  and  n  shows  remarkably  little  change  from  one 
generation  to  another.  Paired  head  spots  are  inherited  in  the  four  groups 
in  85.7,  82.1,  86. 5,  and  89.3  per  cent  of  the  cases,  respectively,  or  in  the  series 
as  a  whole,  in  87  per  cent  of  all  cases  (table  13,  upper  left-hand  square). 
Similarly,  paired  rump  spots  are  inherited  in  the  four  groups  in  7 1 .4,  80, 
80.9,  and  83.1  per  cent  of  the  cases,  respectively,  or  in  the  series  as  a  whole 
(table  13)  in  80.7  per  cent  of  all  cases.  In  both  head  and  rump  spots  the 
inheritance  coefficients  increase  slightly  with  the  progress  of  selection. 

Shoulder  and  side  spots  are  so  frequently  lacking  in  the  Dutch-marked 
series  that  comparison  of  the  per  cents  for  presence  of  these  pairs  would 
have  little  value;  it  is  better  to  examine  rather  the  per  cents  for  absence 
of  these  spots.  Both  shoulder  spots  are  absent  in  the  four  groups  in  58.5, 
59,  41,  and  42  per  cent  of  the  cases,  respectively  (a  decreasing  series) ;  average 
for  the  four  groups,  46.3  per  cent.  Similarly,  side  spots  are  entirely  wanting 
in  the  several  groups  in  50,  34.7,  47.2,  and  44.8  per  cent,  respectively  (again, 
on  the  whole,  a  decrease),  the  average  for  the  four  groups  being  46  per  cent. 

It  is  scarcely  necessary  to  examine  in  detail  other  parts  of  the  several 
tables.  They  show  conditions  for  the  most  part  intermediate  between  those 
seen  in  these  extreme  instances. 

Selection  for  presence  of  spots  (head  and  rump)  and  for  absence  of  spots 
(shoulder)  seems,  in  the  same  series  of  observations,  to  give  contrary  results. 
These  seemingly  inconsistent  facts  are  really  not  inconsistent.  Spots 
desired  present  and  selected  for  have  become  slightly  oftener  present;  spots 
desired  absent  and  selected  against  have  likewise  become  slightly  oftener 
present.  Selection  for  particular  spots  has  had  nothing  to  do  with  bringing 
about  the  result  observed.  The  pigmentation  has  become  somewhat  more 
extensive  in  the  race,  without  our  having  consciously  aimed  to  make  it  so. 
Spots  occur  somewhat  more  numerously  in  the  later  than  in  the  earlier  gen- 
erations of  the  series  (see  table  26).  The  increase,  however,  takes  place  no 
oftener  in  regions  where  increase  is  desired  (head  and  rump)  than  where  it  is 
not  desired  (shoulder) ,  or  where  its  occurrence  is  a  matter  of  indifference  (side) . 

This  result  shows  the  entire  inefficiency  of  selection  in  guinea-pigs  to  fix 
a  coat-pattern  which  is  dependent  upon  the  distribution  of  pigment  in  par- 
ticular spots  upon  the  body. 


IN  RATS  AND  GUINEA-PIGS.  23 

Head  spots  regularly  give  higher  inheritance  coefficients  than  any  other 
set  of  spots,  a  possible  reason  being  that  head  spots  really  include  two 
distinct  pairs  of  pigment  areas,  namely,  eye  spots  and  ear  spots.  Commonly, 
in  the  Dutch-marked  series,  both  of  these  are  pigmented,  in  which  case  it 
makes  no  difference  whether  the  two  are  dealt  with  separately  or  jointly- 
But  sometimes  an  eye  area  may  be  pigmented,  while  the  adjacent  ear  area 
is  unpigmented,  or  vice  versa;  in  such  cases  separate  enumerations  of  eye 
and  of  ear  spots  would  indicate  a  less  amount  of  head  pigmentation  than 
the  joint  reckoning  followed,  for  purposes  of  convenience,  in  this  paper. 
Aside  from  this  consideration,  however,  it  is  certain  that  the  head  region  is 
oftener  pigmented  than  any  other  part  of  the  body. 

HEAD-SPOT  SERIES  (SERIES  H). 

The  data  concerning  spot  inheritance  in  the  head-spot  series  are  contained 
in  tables  15  to  24.  The  entries  are  made  separately  for  the  different  regions 
of  the  body  in  tables  15,  17,  19,  and  21,  for  four  different  groups  of  indi- 
viduals possessing  different  average  amounts  of  head-spot  ancestry.  The 
average  amounts  of  head-spot  ancestry  in  these  several  groups  are  a  half- 
generation,  one  generation,  two  generations,  and  three  generations,  respec- 
tively, and  may  be  expressed  by  the  symbols  //£,  H\t  H2t  and  H3)  as  explained 
in  connection  with  the  Dutch-marked  series.  In  the  group  H\  (table  15), 
one  of  the  parents  of  each  individual  recorded  bore  certain  spots  other  than 
head  spots,  but  in  the  other  three  groups  of  Series//  (tables  17,  19,  and  21), 
none  of  the  parents  bore  spots  other  than  head  spots.  Tables  15,  17,  19, 
and  21  are  combined  in  table  23,  which  shows  the  average  degree  of  in- 
heritance of  particular  spots  in  the  series  as  a  whole. 

As  in  the  case  of  Series  D,  the  entries  for  the  different  regions  of  the  body 
have,  in  the  case  of  each  group  of  individuals,  been  combined  to  get  an  expres- 
sion for  spot  inheritance  in  general,  in  the  particular  group  of  individuals 
under  consideration.  These  combination  entries  are  found  in  tables  16,  18, 
20,  and  22,  for  the  four  groups  considered  separately,  and  combined  into 
one  table  for  the  whole  head-spot  series,  in  table  24. 

In  this  series,  rigid  selection  has  been  exercised  for  head  spots,  and  against 
all  other  spots.  We  will  now  consider  what  the  effects  of  this  two-fold 
selection  are.  Paired  head  spots  are  present  in  the  following  per  cents  of 
the  young  of  parents  possessing  such  spots,  in  the  several  groups:  79.8, 
85.4,  77.1,  and  82.1  per  cent;  average  for  the  series  (table  23),8r.i  per  cent.* 
No  change  is  observed  uniformly  in  the  direction  either  of  increase  or  of 

*In  obtaining  the  average,  each  table  is  weighted  in  proportion  to  the  number  of  individ- 
uals recorded  in  it.  These  are,  for  the  successive  groups,  207,  193, 129,  and  58;  total,  587. 


24  INHERITANCE  OF  COAT-PIGMENTS  AND  COAT-PATTERNS 

decrease  of  head  spots,  but  they  are  on  the  whole  less  common  in  this  series 
than  in  Series  D  (81.1  and  87  per  cent,  respectively). 

However,  shoulder,  side,  and  rump  spots  become  steadily  less  common  in 
the  series,  as  a  glance  at  table  25  will  show.  It  is  doubtful,  however,  whether 
this  is  to  be  regarded  as  an  effect  of  selection  for  elimination  of  specific  areas. 
It  indicates,  rather,  a  decrease  in  the  average  amount  of  pigmentation  pos- 
sessed by  animals  of  this  series,  unwittingly  brought  about  by  the  attempt 
to  eliminate  most  of  the  spots  typically  pigmented  (compare  table  27).  It 
is  significant  that  head  spots,  though  desired  present  and  chosen  equally 
in  Series  D  and  in  Series  H,  occur  more  commonly  in  the  former  than  in  the 
latter.  Animals  of  Series  D  are  more  extensively  pigmented  than  those  of 
Series  H  (compare  tables  26  and  27) ;  the  additional  pigmentation  may  occur 
in  any  of  the  typically  pigmented  areas,  regardless  of  the  areas  which  happen 
to  have  been  pigmented  in  the  selected  ancestors. 

Nevertheless,  we  find  a  more  rapid  falling  off  in  the  pigmentation  of  some 
areas  than  in  that  of  others.  In  Series  H,  shoulder  spots  fall  off  16.9  per 
cent;  side  spots,  7.3  per  cent;  and  rump  spots,  6.4  per  cent;  while  head 
spots,  as  already  noted,  show  little  change.  Now,  selection,  it  will  be  remem- 
bered, was  exercised  for  elimination  of  shoulder,  side,  and  rump  spots  alike, 
but  for  the  retention  of  head  spots.  Accordingly  we  do  not  get  changes 
uniformly  consistent  with  selection.  Regardless  of  selection  for  specific 
spots,  we  get,  when  the  total  amount  of  the  pigmentation  is  decreasing,  a 
decrease  most  rapidly  in  shoulder  spots,  less  rapidly  in  side  spots,  less  rapidly 
still  in  rump  spots,  while  the  head^spots  are  least  affected.  Conversely,  in 
a  series  in  which  the  amount  of  pigmentation  is  increasing,  the  increase  is 
less  marked  in  head  than  in  rump  spots,  and  less  in  side  than  in  shoulder 
spots.  Thus,  in  Series  D,  head  spots  increase  3.6  per  cent,  rump  spots, 
11.7  per  cent;  the  change  in  side  spots  (entire  absence)  is  6.2  per  cent,  of 
shoulder  spots,  16.5  per  cent. 

In  further  support  of  this  same  idea — that  with  increasing  or  decreasing 
pigmentation  changes  occur  more  rapidly  in  some  areas  than  in  others, 
irrespective  of  the  particular  spots  borne  by  the  ancestors — we  may  com- 
pare the  averages  for  Series  D  and  H  (tables  13  and  23).  The  differences 
between  the  two  series  are,  in  frequencies  of  head  spots,  5.9  per  cent;  of 
side  spots  (absent),  20  per  cent;  of  shoulder  spots  (absent),  25.4  per  cent. 
As  regards  rump  spots,  no  comparable  entries  occur  in  either  the  first  or 
the  last  columns  of  tables  13  and  23,  but  an  examination  of  the  middle  column 
(2)  in  both  tables  indicates  that  the  difference  in  rump  spots  in  the  two 
series  is  about  as  great  as  in  head  spots. 

These  facts  are  in  harmony  with  an  observation  made  in  advance  of  the 
statistical  investigation,  that  the  most  persistent  of  all  the  pigmented  areas 
are  the  head  spots,  and  next  in  order  of  persistency  are  the  rump  spots. 


IN   RATS  AND  GUINEA-PIGS.  25 

Our  general  conclusion  from  a  study  of  these  two  series  of  guinea-pigs,  includ- 
ing 1,048  individuals,  is  that  one  can  by  selection  either  increase  or  decrease 
the  extent  of  the  pigmented  areas,  but  it  is  impossible  by  selection  to  fix  this 
pigmentation  in  a  particular  pattern,  retaining  pigment  areas  on  certain  parts 
of  the  body  and  eliminating  them  from  others.  As  the  pigmentation  changes 
in  extent,  under  the  influence  of  selection,  the  various  areas  typically  pigmented 
are  affected  in  the  following  order:  Shoulder,  side,  rump,  and  head,  the  change 
being  greatest  in  the  first-named  and  least  in  the  last-named  area,  irrespective 
of  what  particular  spots  were  present  in  the  selected  ancestors. 

STATISTICAL  ANALYSIS  OF  THE  DATA  FOR  SERIES  D  AND  H. 

The  foregoing  conclusions  may,  we  think,  fairly  be  drawn  from  an  exami- 
nation of  the  tables  as  they  stand.  For  those,  however,  who  place  confidence 
in  the  more  precise  methods  of  statistical  analysis  devised  by  Pearson  and 
others,  it  may  be  more  satisfactory  to  treat  the  tables,  which  have  been 
constructed  for  the  various  groups  of  individuals,  as  correlation  tables,  and 
derive  from  them  the  constants  which  measure  the  variability  of  parents 
and  children  respectively  in  the  several  groups,  and  the  degree  of  correlation 
between  the  two.  Such  constants  are  given,  in  tables  26  and  27,  for  the 
several  groups  of  Series  D  and  H,  respectively. 

From  the  left-hand  columns  of  table  26  we  learn  that,  in  Series  D,  the 
number  of  spots  borne  by  an  individual  increases  from  group  to  group,  in 
the  case  of  both  parents  and  offspring;  but  the  number  of  spots  is,  in  every 
case,  20  to  26  per  cent  greater  in  the  offspring  than  in  their  parents.  This 
indicates  a  tendency  for  the  offspring  to  become  pigmented  in  regions  which 
lacked  pigment  in  their  selected  parents. 

Table  27  shows  the  existence  of  a  similar,  but  still  stronger,  tendency  in 
Series  H,  the  offspring  bearing  50  to  105  per  cent  more  spots  than  their 
parents.  This  tendency  may  be  considered  regression,  a  tendency  to  return 
to  a  condition  of  more  widely  distributed  pigmentation,  and  to  acquire 
spots  where  the  selected  ancestors  lacked  them.  It  does  not  imply  that  the 
young  bear  more  pigment  than  their  parents.  The  regression  is  stronger 
the  more  reduced  the  pigmentation  of  the  parents,  as  we  might  expect. 
This  is  seen  to  be  true  both  within  Series  H  and  in  that  series  as  a  whole 
compared  with  Series  D. 

The  standard  deviation  from  the  average  number  of  spots  is  for  the 
offspring  a  very  constant  quantity,  being  close  to  3.6  in  both  Series  D  and 
Series  H.  This  indicates  no  change  in  the  variability  in  number  of  spots 
as  a  result  of  selection,  or  as  a  consequence  of  change  in  the  number  of  pig- 
mented areas.  The  parents  show  throughout  Series  H  and  in  Group  D%  of 
Series  D  a  less  standard  deviation  than  their  offspring.  This  fact,  however, 


26  INHERITANCE  OF   COAT-PIGMENTS   AND  COAT-PATTERNS 

has  for  our  purposes  little  significance,  since  the  parents  are  selected  indi- 
viduals, while  their  offspring  are  not.  The  same  may  be  said  of  the  coeffi- 
cients of  variability  for  the  parents.  For  the  offspring,  the  coefficient  of 
variability  (ratio  of  standard  deviation  to  mean,  times  100)  decreases  slightly 
in  Series  D,  and  increases  in  Series  H.  This  must  not  be  interpreted  as 
signifying  a  change  in  variability  of  an  opposite  nature  in  the  two  series. 
It  is  due  entirely  to  the  changes  in  the  mean  (average  number  of  spots), 
these  changes  being  opposite  in  nature  in  the  two  series.  If  the  mean  re- 
mained constant  throughout  each  series,  the  amount  of  variability  indicated 
by  the  coefficient  of  variability  would  likewise  remain  constant ;  for  we  have 
seen  that  the  standard  deviation  is  constant,  irrespective  of  changes  in  the 
mean  and  irrespective  of  the  number  of  generations  during  which  selection 
has  been  in  progress.  Standard  deviation  is,  therefore,  a  better  measure 
of  variability  than  the  coefficient  of  variability  in  these  series. 

The  coefficient  of  correlation  (r)  has  little  real  significance  in  tables  26 
and  27.  This  constant  is  relatively  small  in  the  Dutch-marked  series  and 
grows  smaller  as  selection  progresses,  whereas  in  the  head-spot  series  it  is 
relatively  large  and  grows  larger  with  the  progress  of  selection.  Selection 
does  not  have  an  opposite  effect  in  the  two  series  upon  the  inheritance  of 
a  coat-pattern.  The  whole  effect  is  due  to  change  in  the  amount  of  the 
pigmentation,  not  to  its  distribution.  When  the  amount  of  pigmented 
surface  is  small,  its  distribution  in  certain  spots  (head  and  rump)  is  more 
certain  and  the  correlation  rises  (Series  //).  When,  on  the  other  hand,  the 
pigmented  surface  is  large  (Series  D,  table  26),  its  distribution  is  less  certain 
and  the  correlation  is  low;  it  becomes  lower  as  the  pigmentation  increases, 
in  spite  of  progressive  selection. 

The  statistical  analysis  confirms  the  conclusions  previously  drawn  from 
our  observations.  Selection  is  powerless  to  fix  a  particular  coat-pattern 
not  dependent  upon  amount  of  pigmentation.  It  is  as  powerless  to  decrease 
the  variability  in  number  of  spots  as  to  fix  any  pattern  formed  by  them. 

NOSE-SPOT  SERIES  (SERIES  N). 

It  is  only  in  the  case  of  the  nose-spot  series  (Series  N)  that  we  are  able  to 
detect  influence  of  selection  in  fixing  a  pigment-pattern  among  guinea-pigs. 
Two  individuals  showing  this  rather  striking  variation  in  pattern  were  figured 
by  Castle  (105,  pi.  6);  another  is  shown  in  pi.  2,  fig.  i,  of  this  paper.  The 
starting-point  of  the  series  of  selections  for  the  fixation  of  this  pattern  was 
the  male  1989°  shown  in  fig.  12,  pi.  6  (Castle,  105).  Besides  the  nose  spot 
shown  in  the  figure,  this  animal  bore  a  distinct  right  shoulder  spot,  not 
visible  in  the  figure.  This  animal  produced  a  considerable  number  of  young 
with  nose-spot  markings  similar  to  his  own,  as  will  be  seen  from  an  examina- 
tion of  table  28.  As  a  result  of  matings  with  animals  which  did  not  bear 


IN   RATS   AND  GUINEA-PIGS.  27 

nose  spots,  he  produced  four  individuals  with  nose  spots  and  fourteen  with- 
out nose  spots,  or  22.2  per  cent  of  nose-spot  (NS)  young.  When  he  was 
mated  with  these  nose-spot  young  (N  S%)  or  with  nose-spot  individuals 
produced  in  other  experiments  but  not  of  nose-spot  ancestry,  and  so  desig- 
nated N  S0,  he  produced  a  much  higher  percentage  of  nose-spot  young,  viz, 
53.6  per  cent  by  AT  So  mothers,  and  61.5  per  cent  by  N  S$  mothers.  In 
one  case  he  was  mated  with  his  NSi  daughter  by  an  N  S0  mother,  from  which 
mating  there  resulted  three  nose-spot  young  and  two  otherwise  marked,  or  60 
per  cent  N  S.  The  number  of  young  in  this  last  experiment  is  too  small 
to  be  very  significant  quantitatively,  but  those  in  the  other  matings  are 
large  enough.  They  show  an  increase  in  the  per  cent  of  NS  young  with  an 
increase  in  the  amount  of  NS  ancestry,  and  strongly  suggested  the  possi- 
bility of  a  still  further  increase  by  continued  selection.  Such  increase, 
however,  has  not  up  to  this  time  been  realized.  Five  sons  or  grandsons  of 
c?  1989°  have  been  quite  extensively  tested  and  the  same  is  true  of  one 
individual  ((^5595,  NSi,  table  29)  descended  from  a  brother  of  1989°.  Of 
these  six  males,  two  have  records,  about  equaling  in  production  of  nose- 
spot  young  the  record  of  c?  1989*,  but  the  records  of  the  remaining  four 
are  inferior  to  his.  The  two  males  with  the  best  records  are  ^5652  (table  29), 
a  son  of  cT  1989*,  and  cT  5669  (table  29),  a  grandson  of  c?  1989°  and  son  of 
c?5i5i  (table  29).  Neither  of  these  males,  however,  has  produced  a  large 
number  of  young  (the  former  16,  the  latter  10),  and  so  too  much  importance 
must  not  be  attached  to  the  per  cent  results.  Of  all  the  six  males,  5151 
alone  has  produced  more  than  fifty  young.  By  NSQ  and  AT  Si  mothers  alike, 
about  40  per  cent  of  his  young  bear  nose  -spots.  His  father's  record  by  the 
same  group  of  mothers  was  about  15  per  cent  better  (see  table  28). 

The  collective  results  given  by  the  six  males  enumerated  in  table  29  are 
shown  in  table  30.  They  indicate  that  in  the  long  run  a  higher  percentage 
of  NS  young  is  produced  by  NSi  than  by  NS0  sires,  but  neither  group 
gives  as  high  a  percentage  as  the  original  nose-spot  male,  1989°  (table  28). 

The  nose-spot  mothers  employed  in  these  experiments  are  much  more 
numerous  than  the  males,  but  because  the  number  of  young  produced  by 
any  one  of  them  is  in  no  case  greater  than  fifteen,  the  results  are  not  given 
for  the  mothers  individually,  but  only  in  collective  form  (table  30,  last  part) . 

The  collective  results  for  the  females,  like  those  for  the  males,  show  an 
increase  in  the  percentage  of  nose-spot  young  with  increase  in  the  amount 
of  nose-spot  ancestry.  In  other  words,  the  character  is  apparently  inherited 
feebly  through  both  the  male  and  the  female  lines,  and  is  being  gradually 
fixed  by  selection.  The  process,  however,  is  a  slow  one.  After  two  gener- 
ations of  selection,  the  inheritance  coefficient  is  no  higher  than  in  the  original 
male,  1989".  Whether  it  can  ultimately  be  made  higher  remains  an  open 
question. 


28  INHERITANCE   OF   COAT-PIGMENTS   AND   COAT-PATTERNS 

The  objection  may  be  made  that  a  nose  spot  as  such  is  not  inherited, 
any  more  than  an  eye  spot  or  a  rump  spot  (see  page  25) ;  that  in  a  spotted 
race  a  certain  average  amount  of  pigmentation  is  an  inherited  condition, 
but  the  distribution  of  this  pigment  is  wholly  a  matter  of  chance.  To  test 
this  point,  comparison  has  been  made  with  the  young  of  certain  sires  in  Series 
H  (table  31).  In  the  case  of  the  litters  recorded  in  this  table,  neither  parent 
bore  a  nose  spot,  nor  came  from  a  nose-spot  family.  The  extent  of  the  pig- 
mentation of  the  parents  is  measured  roughly  by  the  number  of  the  typical 
spot-areas  which  were  pigmented  (see  columns  2,  3,  4,  7,  and  10,  table  31). 
For  comparison  with  the  nose-spot  series,  see  columns  3  and  6  of  tables  28, 
29,  and  30.  It  will  be  observed  that  the  extent  of  the  pigmentation  is  similar 
in  the  nose-spot  and  head-spot  series,  being  in  both  cases  close  to  an  average 
of  three  spots  to  an  individual.  The  percentage  of  nose-spot  young,  how- 
ever, is  much  lower  in  the  eye-spot  than  in  the  nose-spot  series,  being  16.3 
per  cent  in  the  former  as  compared  with  40  to  50  per  cent  in  the  latter. 
That  the  nose-spot  marking  does  occur  quite  frequently  among  spotted 
guinea-pigs  not  selected  for  that  character  is  shown  clearly  by  table  31. 
That  its  occurrence,  however,  is  more  frequent  when  selection  is  made  for 
the  character  is  evident  from  tables  28  to  30. 

In  table  31,  as  in  tables  28  to  30,  it  will  be  observed  that  nose-spot  young 
have  fewer  other  spots  than  do  their  brothers  and  sisters  that  lack  nose 
spots.  The  average  difference  is  from  half  to  two -thirds  of  a  spot.  This 
means  that  a  nose  spot  takes  the  place,  to  some  extent,  of  pigmentation 
elsewhere,  and  it  is  in  part  a  matter  of  chance  whether  the  pigmentation  is 
located  on  the  nose  or  elsewhere.  But  chance  is  not  the  only  element  (if 
we  may  so  speak  of  chance)  entering  into  the  matter,  for  nose-spot  parents 
clearly  produce  more  nose-spot  young  than  do  other  parents  transmitting 
a  like  amount  of  pigmentation.  How,  then,  are  we  to  account  for  the  fact 
indicated  by  our  observations  that,  while  other  patterns  are  unfixable,  the 
nose-spot  pattern  is  in  part  at  least  fixable?  At  present  we  can  not  account 
for  it,  but  a  consideration  of  familiar  facts  concerning  mammalian  develop- 
ment may  help  us  in  shaping  a  hypothesis. 

The  production  of  hair  and  skin  pigments  in  guinea-pigs  is  the  exclusive 
function  of  the  ectoderm,  as  shown  by  Leo  Loeb  ('97)  and  confirmed  by 
Castle  (-.05).  In  spotted  guinea-pigs  the  limits  of  the  pigment  spots  are 
very  precisely  defined  at  birth  and  these  limits,  so  far  as  we  have  been  able 
to  observe,  are  never  subsequently  transgressed  in  the  slightest  degree. 
Areas  which  are  white  at  birth  remain  white  ever  afterward;*  areas  which 

*We  leave  out  of  consideration  for  the  present  the  "peripheral"  pigmentation  which 
albinos  as  well  as  spotted  animals  may  possess  (see  Castle,  105).  This  is  not  fully  developed 
at  birth. 


IN   RATS  AND  GUINEA-PIGS.  29 

are  black  at  birth  remain  black;  those  which  are  yellow  remain  yellow; 
and  those  which  contain  at  birth  black  and  yellow  hairs  interspersed  remain 
in  that  condition  ever  afterward.  This  indicates  that  every  portion  of  the 
epidermis  has  its  pigment-forming  capacity  early  and  finally  differentiated. 
In  spotted  animals  the  capacity  to  form  hair  and  skin  pigments  is  trans- 
mitted only  to  certain  portions  of  the  epidermis.  Our  statistical  studies 
make  it  clear  that  guinea-pigs  and  rats  (and  probably  other  spotted  mam- 
mals also)  transmit  with  a  good  deal  of  constancy  definite  amounts  of 
pigmented  surface,  but  that,  in  guinea-pigs  at  least,  the  distribution  of  this 
pigmentation  over  the  body  is  not  strictly  localized  in  the  germ.  A  certain 
amount  of  pigment,  apparently,  is  handed  over  to  the  epidermis,  but  it 
seems  to  be  to  some  extent  a  matter  of  chance  upon  what  part  of  the  body 
this  pigmented  epidermis  finally  comes  to  lie.  It  is  not,  however,  entirely  a 
matter  of  chance.  It  is  almost  certain  that  the  pigment  will  lie  chiefly  on 
the  dorsal  surface,  and  if  the  pigmentation  is  not  extensive,  it  will  be  re- 
stricted to  one  or  more  of  the  regions  which  we  have  designated  eye,  ear, 
shoulder,  side,  and  rump  areas,  all  of  which  are  paired  and  frequently  separated 
one  from  another  by  intermediate  unpigmented  areas.  In  other  cases, 
even  when  adjacent  areas  are  confluent,  they  show  their  essential  distinct- 
ness by  sharp  differences  in  color.  Besides  the  paired  areas  mentioned, 
there  is  an  unpaired  area  at  either  end  of  the  body.  The  anterior  one  we 
have  designated  nose  spot,  the  posterior  one  might  be  called  a  tail  spot, 
though  in  the  guinea-pig  it  is  scarcely  distinguishable  from  the  rump  spots, 
because  there  is  no  external  tail.  The  distinct  pigment  spots  are  derivatives, 
doubtless,  of  individual  blastomeres  set  apart  early  in  development  for  the 
production  of  the  epidermis.  We  know  that  in  birds  and  mammals  the 
epidermis  is  first  differentiated  along  either  side  of  the  primitive  streak. 
The  ectoderm  along  the  middle  of  the  primitive  streak  sinks  down  to  form 
the  neural  canal,  then  the  divided  right  and  left  halves  of  the  epidermis 
come  together  above  it,  while  anterior  and  posterior  to  the  neural  invagina- 
tion  the  right  and  left  halves  of  the  epidermis  have  been  from  the  beginning 
continuous.  Very  likely  the  nose-  and  tail-spot  regions  correspond  with 
these  regions  of  original  continuity  of  the  right  and  left  halves  of  the  epider- 
mis, while  the  paired  areas  are  formed  out  of  the  epidermis  lateral  to  the 
neural  in  vagina  tion.  The  epidermis  of  the  ventral  side  of  a  bird  or  mam- 
mal is  developed,  we  know,  later  than  the  dorsal  portions,  and  in  spotted 
individuals,  apparently,  the  amount  of  pigment  inherited  is  insufficient 
ordinarily  to  extend  out  over  the  blastoderm  into  this  region.  Accordingly 
the  pigment  is  restricted  to  the  portions  of  the  epidermis  first  differentiated, 
i.  e.}  adjacent  to  the  primitive  streak.  In  most  cases,  it  is  insufficient  to 
cover  more  than  portions  even  of  that. 


30  INHERITANCE  OF  COAT-PIGMENTS  AND  COAT-PATTERNS 

On  this  interpretation,  the  difference  between  a  black-eyed  white  and  a 
pink-eyed  white  (or  albino)  guinea-pig  is  this :  The  neural  plate  of  the  former 
receives  a  pigment  potentiality  from  the  ectoderm  which  is  invaginated  to 
form  the  neural  canal  (out  of  which  the  retina  of  the  eye  is  later  formed) ; 
the  albino  does  not  receive  the  pigment  potentiality  into  the  neural  plate, 
simply  because  that  potentiality  is  absent  from  the  entire  ectoderm.  In 
the  black-eyed  white  guinea-pig  no  part  of  the  epidermis  is  pigmented, 
because  all  the  color-bearing  ectoderm  has  gone  into  the  neural  invagina- 
tion  and  none  is  left  outside  for  the  epidermis.  When  a  very  little  is  left 
outside,  it  usually  is  found  either  upon  an  ear,  upon  one  side  of  the  fore- 
head, or  about  the  eye  (see  Castle  105),  facts  which  indicate  with  consider- 
able clearness  what  relation  the  epidermis  of  the  head  region  bore  to  the 
anterior  part  of  the  neural  invagination,  and  support  the  view  that  the  nose 
spot  arises  out  of  epidermis  originally  anterior  to  the  neural  plate.  Black- 
eyed  white  guinea-pigs  usually  produce  offspring  with  one  or  more  spots  on 
the  body,  *.  e.,  the  amount  of  pigment  which  they  transmit  is  commonly  large 
enough  to  extend  lateral  or  anterior  to  the  neural  plate  and  cause  epidermal 
spots  as  well  as  pigmented  eyes.  Thus  a  pair  of  black-eyed  white  guinea- 
pigs  has  recently  (Nov.,  1906)  produced  one  black-eyed  white  young  and  two 
with  nose  spots  and  an  ear  spot  each.  In  spotted  guinea-pigs  the  pigment 
is  apparently  always  centralized  about  the  primitive  streak.  It  invariably 
passes  into  the  neural  canal  (if  any  pigment*  is  inherited  at  all),  so  that 
the  eyes  are  pigmented;  usually  it  extends  out  also  into  the  epidermis 
bordering  on  the  neural  plate,  but  its  limits  are  variable.  When  the 
amount  of  epidermal  pigment  is  small,  it  is  found  most  often  upon  the  head, 
adjacent  to  the  portion  of  the  neural  plate  out  of  which  the  eyes  were 
formed. 

If  we  could  follow  through  the  period  of  cleavage  the  history  of  the 
pigment-forming  substance  of  the  fertilized  egg,  we  should  probably  find 
that  in  spotted  guinea-pigs  this  is  distributed  to  certain  blastomeres  in 
the  animal  hemisphere  of  the  egg.  Apparently  it  is  to  some  extent  a 
matter  of  chance  what  blastomeres  receive  the  pigment,  but  the  anterior 
part  of  the  neural  invagination  is  sure  to  receive  pigment,  and  the  adjacent 
portions  of  the  skin  are  more  likely  than  any  other  regions  to  do  so.  In 
nose-spot  animals,  pigment  is  evidently  distributed  to  the  most  anterior 
epidermal  cells  which  take  part  in  forming  the  dorsal  surface  of  the  animal. 
If,  as  our  observations  indicate,  the  condition  is  to  some  extent  hereditary, 
the  pigment  potentiality  must  be  localized  in  both  egg  and  sperm  in  that 

*See  footnote,  page  28. 


IN   RATS  AND  GUINEA-PIGS.  31 

part  of  the  germ-cell  which  gives  rise  to  the  anterior  part  of  the  primitive 
streak.  In  selecting  for  the  nose-spot  character,  accordingly,  we  have 
simply  selected  for  extreme  anterior  localization  of  a  reduced  amount  of  pig- 
mentation. Selection  for  localization  less  far  forward  is  apparently  less 
effectual,  probably  because,  in  that  part  of  the  germ,  a  cell-division,  which 
would  involve  unequal  distribution  of  the  pigment  potentiality  to  the 
daughter  cells,  might  carry  the  potentiality  either  forward  or  backward  in 
the  embryo,  whereas  original  extreme  anterior  localization  could  result  in 
its  transportation  only  forward  to  the  most  anterior  part  of  the  embryo. 
This,  while  a  purely  hypothetical  explanation,  is  offered  as  a  suggestion 
to  the  embryologist,  who  some  day,  perhaps,  will  be  able  to  identify  in  the 
germ  and  trace  through  its  various  stages  of  development  the  substance 
or  substances  on  the  presence  of  which  pigment  formation  depends. 

If  anterior  localization  of  pigment  is  possible  through  selection,  it  would 
seem  that  posterior  localization  should  likewise  be  possible.  We  began  at 
one  time  a  series  of  experiments  to  test  this  point,  but  the  results  given  by 
the  first  selected  generation  were  so  unpromising  that  the  experiment  was 
abandoned.  It  may  be  pointed  out,  however,  that  one  might  expect  fixa- 
tion of  such  a  pattern  to  be  more  difficult,  because,  like  the  Dutch-marked 
pattern,  it  involves  a  double  selection,  viz,  selection  for  the  posterior  local- 
ization of  pigment  sought  (tail  spot)  and  simultaneously  and  unavoidably 
selection  for  the  anterior  localization  of  pigment  represented  in  the  black 
eyes  of  all  spotted  individuals. 

The  explanation  which  has  been  offered  for  the  distribution  of  pigment  in 
partial-albino  guinea-pigs  will  apply  equally  well  to  the  case  of  rats,  with 
this  difference:  In  rats  the  body  is  more  elongated  than  in  the  guinea-pig^ 
and  pigment-reduction  affects  at  first  rather  the  lateral  than  the  longitudinal 
distribution  of  pigment.  The  back-stripe  first  becomes  narrower,  then 
becomes  interrupted,  and  finally  drops  out  altogether,  leaving  pigment 
only  on  the  head  and  usually  also  sparingly  on  the  tail  (see  pi.  i).  This 
condition  corresponds  with  that  condition  of  the  guinea-pig  in  which  head 
and  tail  (or  rump)  spots  only  are  present,  a  condition  very  common  in 
Series  H,  as  we  have  seen.  As  the  hood  becomes  reduced  in  extent,  the 
white  areas  first  extend  forward  ventrally  to  the  mouth,  then  in  the  median 
dorsal  line  between  the  shoulders,  and  simultaneously  a  white  spot  appears 
in  the  forehead  (see  pi.  i,  fig.  3).  Further  reduction  than  this  has  not  yet 
been  obtained,  but  it  is  evident  that  the  pigmented  areas  are  becoming 
restricted  toward  the  sides  of  the  head  adjacent  to  the  eyes,  precisely  as  in 
guinea-pigs. 


32  INHERITANCE   OF   COAT-PIGMENTS   AND   COAT-PATTERNS 

CONCLUSIONS. 

The  results  of  selection  brought  to  bear  upon  the  coat-pattern  are  seem- 
ingly very  different  in  rats  and  in  guinea-pigs,  yet  a  careful  analysis  of  the 
facts  shows  the  results  to  be  not  so  dissimilar  in  the  two  cases  as  they  at 
first  thought  appear. 

In  both  rats  and  guinea-pigs  we  can  by  selection  increase  or  decrease  at 
will  the  average  extent  of  the  pigmented  areas.  In  both  rats  and  guinea- 
pigs  the  extent  of  the  pigmented  areas  varies  continuously,  and  out  of  these 
continuous  variations  permanent  modification  of  the  pigmentation  can  be 
secured. 

Reduction  in  the  total  amount  of  the  pigmentation  is  attended  in  rats 
by  restriction  of  the  pigment  to  very  definite  areas,  whereas  in  guinea-pigs 
it  may  be  distributed  in  any  or  in  all  of  a  series  of  spots.  Herein  lies  the 
whole  difference  between  the  two  cases.  When  in  rats  we  select  for  reduced 
pigmentation,  we  get  animals  with  a  narrow  or  interrupted  back-stripe  and 
with  a  less  extensive  hood;  when  in  guinea-pigs  we  make  a  similar  selection, 
we  get  animals  with  fewer  or  less  extensive  spots.  We  can  not  in  guinea- 
pigs  decide  arbitrarily  which  areas  shall  be  pigmented  (except,  possibly,  in 
the  case  of  nose  spots) ,  any  more  than  in  rats  we  can  at  the  same  time 
increase  the  extent  of  the  hood  and  decrease  that  of  the  back-stripe. 

In  rats,  we  have  as  a  result  of  pigment  reduction  a  series  of  coat-patterns, 
each  breeding  true  within  certain  limits;  in  guinea-pigs,  the  fluctuation  in 
the  extent  of  the  pigmented  areas  is  probably  no  greater  than  in  rats,  but 
because  the  pigmented  areas  do  not  disappear  in  as  definite  an  order  during 
pigment  reduction,  we  have  no  constant  coat-patterns.  Nevertheless  there 
is  every  reason  to  suppose  that  different  degrees  of  pigmentation  are  in- 
herited in  Mendelian  fashion  in  guinea-pigs,  precisely  as  they  are  in  rats. 
If  the  pigment  reduction  followed  a  definite  course  in  guinea-pigs,  as  it  does 
in  rats,  this  would  be  easily  recognizable  in  the  coat-pattern.  As  it  is, 
measurement  of  the  extent  of  the  pigmented  areas  would  be  necessary  to 
make  it  apparent.  This  we  have  not  undertaken  to  do  in  the  case  of  guinea- 
pigs;  we  have  merely  taken  account  of  the  regions  pigmented,  not  of  their 
extent.  This  probably  explains  in  part  why  regression  is  observed  in  the 
selection  experiments  with  guinea-pigs,  but  not  with  uniformity  in  those 
with  rats.  In  guinea-pigs  we  attempted  by  selection  to  restrict  the  number 
of  the  pigmented  areas ;  this  was  found  to  be  impossible  except  as  it  occurred 
incidentally  to  reduction  in  the  total  amount  of  pigmentation.  The  regres- 
sion occurred  in  number  of  pigmented  areas,  not,  so  far  as  we  know,  in  the 
total  amount  of  the  pigmentation.  We  have  no  doubt,  however,  that  such 
regression  would  be  found  to  occur  in  cases  in  which  extreme  variates  were 
selected.  We  have  found  it  so  in  selecting  black-eyed  white  guinea-pigs, 
those  with  no  pigment  except  in  the  eye.  Almost  invariably  the  young  of 


IN  RATS  AND  GUINEA-PIGS.  33 

such  animals  have  borne  more  pigment  than  did  their  parents.  A  similar 
result  would  doubtless  follow  selection  of  self-pigmented  rats  obtained  from 
Irish  parents.  No  doubt  many  of  the  young  would  bear  some  white  fui. 
With  selection  of  less  extreme  variates,  regression  less  extreme  may  possibly 
occur,  though  our  statistical  observations  do  not  show  any  regression  in 
the  case  of  rats.* 

If  regression  does  occur,  can  we  with  propriety  consider  the  effects  of  selec- 
tion permanent?  De  Vries  has  answered  this  question  in  the  negative  on 
the  basis  of  his  selection  experiments  with  maize,  striped  flowers,  double 
buttercups,  and  other  similar  material.  It  seems  to  us,  however,  that  the 
answer  should  be  qualified.  The  final  result  will  depend  upon  the  amount 
and  the  persistency  of  the  regression.  In  De  Vries's  experiments  with  maize, 
as  in  those  of  Fritz  Miiller  ('86),  the  regression  grows  less  with  each  selection. 
If  this  continued,  the  regression  should  ultimately  become  a  negligible  quantity. 
After  repeated  selection  for  a  desired  extreme  condition,  the  race  should 
become  stable  at  a  condition  only  a  little  less  extreme  than  that  selected. 

De  Vries's  fine  series  of  selection  experiments  with  the  buttercup  (Ranun- 
culus bulbosus)  seems  to  the  writers  scarcely  to  justify  the  conclusion  that 
selection  has  no  permanent  effects.  Starting  with  a  one-sided  or  "half- 
Galton"  variation  curve,  with  a  range  from  the  modal  number,  5,  upward 
to  13,  De  Vries  was  able  by  selection  for  an  increased  number  of  petals  to 
raise  the  mode  to  u,  the  average  to  8.6,  and  the  upper  limit  of  variation  to 
31,  and  to  obtain  a  two-sided,  or  Galtonian,  variation  curve  with  only  a 
moderate  amount  of  skewness,  and  with  greatly  diminished  regression.  All 
this  was  accomplished  within  five  generations. 

We  consider  the  selection  question  still  an  open  one.  Further  experiments 
and  longer  continued  ones  are  needed.  Our  own  observations,  so  far  as 
they  go,  and  those  of  Fritz  Miiller  and  De  Vries,  lead  us  to  think  that  selection 
is  a  most  important  factor,  not  only  in  the  isolation  of  discontinuous  varia- 
tions, but  also  in  their  production. 

Further,  we  are  far  from  convinced  that  all  evolutionary  progress  is  to  be 
attributed  to  discontinuous  variations,  any  more  than  to  Mendelian  inher- 
itance. The  distinction  between  continuous  and  discontinuous  variations 
is  a  useful  one,  just  as  that  between  alternative  and  blending  inheritance, 
but  a  sharp  line  of  division  can  be  drawn  in  neither  case.  The  hooded  and 
Irish  coat-patterns  of  rats  are  recognized  discontinuous  variations,  alter- 
native in  inheritance,  yet  our  lot  M  of  hooded  rats  is  as  nearly  intermediate 
between  typical  hooded  and  typical  Irish  rats  as  anything  that  can  well  be 
imagined.  The  coat-patterns  of  fancy  rats,  though  discontinuous  as  they 
ordinarily  occur,  can  be  transformed  into  continuous  variations.  Concerning 

*April,  1907.  In  this  year's  experiments  we  are  getting  some  evidence  of  the  expected 
regression. 


34 


INHERITANCE  OF  COAT-PIGMENTS  AND  COAT-PATTERNS 


the  hooded  and  Irish  patterns,  Doncaster  (:o6),  after  an  extended  experience, 
says  (p.  216):  "Only  once  have  I  had  any  hesitation  in  classing  a  rat  as 
belonging  to  one  or  the  other."  Yet  we  have  seen  that  by  cross-breeding 
and  selection  these  same  discontinuous  groups  can  be  made  continuous  by 
the  production  of  any  desired  number  of  intermediate  groups,  each  varying 
continuously  about  a  different  mode. 

Again,  though  the  inheritance  is  clearly  Mendelian,  when  hooded  and  Irish 
rats  are  crossed,  the  gametes  formed  by  cross-breds  are  not  pure,  but  modified, 
each  extracted  pattern  being  changed  somewhat  in  the  direction  of  that 
pattern  with  which  it  was  associated  in  the  cross-bred  parent.  This  means 
simply  that  the  inheritance,  though  in  the  main  alternative,  is  to  some  extent 
blending. 

Since  it  is  impossible  to  make  a  sharp  distinction  between  continuous  and 
discontinuous  variations,  as  well  as  between  blending  and  alternative  inherit- 
ance, it  is  fallacious  to  assign  all  evolutionary  progress  to  one  sort  of  variation 
or  to  one  sort  of  inheritance. 

TABLES. 

TABLE  i. — Actually  different  classes  into  which  we  may  expect  the  five  visibly  different 

classes  of  albino  and  partial-albino  rats  to  jail. 
[Twenty-two  of  the  twenty-seven  classes  enumerated  have  been  proved  to  exist.] 


Classes  visibly  different. 

Gray  Irish. 

Gray 
hooded. 

Black 
Irish. 

Black 
hooded. 

Albino. 

r 

i  GI 

i  GH 

i  BI 

i  BH 

iW 

"BH] 

. 

2  GKW) 

2  GH(W) 

2  BI(W) 

2  BH(W) 

i  W 

BI] 

2  GI(B> 

2  GH(B) 

2  BI(H) 

i  W 

GH] 

Classes 
expected 

2  GI(H) 
4GI(W.B) 
4GI(W.H) 
4  GI(BII) 

4GH(W.B) 

4  BIcW.H) 

i  W 

2    W 
2  W 

2  W[ 

GIJ 
"B-IH] 
G-IH] 
GB-H] 

. 

SGI(W.BH) 

. 

• 

2  W[ 

4w 

GB-I] 
GB-IH] 

Total... 

27 

9 

9 

3 

16 

B,  black;  G,  gray;  H,  hooded;  I,  Irish;  W,  total  albinism.  Symbols  indicating  unseen 
recessive  characters  are  placed  within  parentheses  (  ),  those  indicating  characters  latent 
in  albinos  are  placed  within  brackets  [  ]. 

When  two  symbols  only  are  used  the  first  refers  to  color,  the  second  to  coat -pattern. 

When  more  than  two  symbols  stand  together  within  brackets,  those  which  refer  to 
color  are  placed  at  the  left  of  a  hyphen,  those  referring  to  pattern  at  its  right. 

Total  albinism  is  indicated  by  W.  When  albinism  is  recessive  with  other  characters 
in  pigmented  individuals,  the  W  will  be  separated  by  a  period  from  the  symbols  designating 
the  other  characters. 

The  numerals  prefixed  to  the  several  class-designations  indicate  the  expected  frequencies 
of  the  classes  when  individuals  are  mated  inter  se  which  have  the  characters  indicated  by 
the  designation  GI  (W.BH). 


IN  RATS  AND  GUINEA-PIGS. 


35 


2. — Expected  and  observed  distribution  of  young  produced  by  partial- 
albino  rats  mated  inter  se. 
[Abbreviations  as  in  table  1.] 


Mating  No. 

Nature  of  mating. 

Proportions  of  off- 
spring expected  on 
Mendelian   hypothesis. 

Observed  result 
(expected  result,  italic). 

GI 

GH 

BI. 

BH. 

W. 

GI. 

GH. 

BL 

i 

2.4 

5 
J 

6.J 

13 
15 
o 

J.* 

5 
4 
o 
o 

0 

0 
0 

o 

0 
0 
0 

o 

26 

*7.6 

5 
7 

BH. 

W. 

I 

2 

3 
4 
5 
6 

7 
8 

9 

10 

ii 

12 
13 
14 

GI(W.BH)  bred  inter  se.. 
GI(W.BH)xGM(W.BH) 
GI(W.BH)XBI(W.BH).. 
GI(W.BH)X  BH(W)  
GI(W.BH)X  BH  

27 
9 
9 

3 

i 

i 

0 
0 
0 
0 
0 
0 
0 

9 
9 
3 
3 
i 

0 

i 
3 

9 
3 

i 
o 

0 
0 

9 
3 
9 
3 
i 
i 

0 

o 
o 

0 
0 

9 
3 

i 

i 

3 
3 
3 
3 

i 

0 
0 

i 
3 
3 

i 

3 
3 
i 

16 
8 
S 
4 

0 
0 
0 
0 

4 

2 
0 

4 

2 
0 

(  ii 
1    72 

1    S 
I    9 

i    8 
1    6.3 

i13 

(  '5 
(    6 

1    3-* 

\\ 

<    8 
\    9 

\    ° 
1     o 

i  ° 

\    o 

i  ° 

i      0 

(     0 

1    o 
1    o 

1    o 

(      0 

1    o 

!  ° 

i    o 

2 

2.4 

10 

9 

i 

2  I 

16 
'5 
3 

3-* 

0 
0 
IO 

9 

18 

21 

«4 

12.4 

H 
'3-9 
4 
J-5 
o 
o 

0 

o 
o 
o 

0.J 

2 
J 

4 

2.1 
24 

'5 
4 
,?•* 

0 

o 
o 
o 

10 

7 

2 

*' 
12 

'39 
3 
J5 
ii 
?.j 

20 

18 
9 
7 

2 
^^ 

I 
5 

5-5 
H 

^0 

o 
o 

0 

o 

0 
0 
0 

o 
6 

f-5 
ii 

92 

0 

o 

12 
122 

U 
12 
0 
0 

GI(W  B)X  BI(H)     .     ... 

GI(H)XBH(VV)  

GH(W  B)xGH(B)  

GH(W.B)  bred  inter  se... 
GH(W  B)xBH(W) 

GH(W  B)x  BH       

BI(W.H)bred  inter  se.... 
BI(W  H)X  BH(W) 

BI(H)XBH  

57 
5J-6 

92 

9'-  5 

74 

86.4 

102 

S6.8 

70 
767 

INHERITANCE  OF  COAT-PIGMENTS  AND  COAT-PATTERNS 


3. — Expected  and  observed  distribution  of  young  produced  by  partial- 
albino  rats  mated  uith  albinos. 
[Abbreviations  as  in  table  1.] 


0 

tJD 

c 

H 
% 

Nature  of  mating. 

Proportions  of  off- 
spring expected  on 
Mendelian  hypothesis. 

Observed  result 
(expected  result,  italic). 

GI. 

GH. 

BI. 

BH. 

W. 

GI. 

GH. 

BI. 

BH. 

W. 

15 

16 

17 

18 

19 

20 
21 
22 
23 

GI(W.BH)X\V[BH]  

i 

0 
0 

o 

0 

3 
o 

0 
0 

i 

i 
o 

3 
o 
i 

0 

o 

0 

o 

3 

o 

I 

i 

i 
i 
i 
i 
i 
o 
i 

4 

2 
0 
O 
0 

8 

4 
i 

2 

11. 

{o 
(    o 

t    ° 

\    ° 
1    o 

\: 

\5<, 

f    o 
'(    o 

is 

1° 

L 

6 
II 

1J 

11.5 

0 

o 

2 

*>4 
o 
o 

8 

65 
o 

0 

6 

8.1 
o 
o 
o 

0 

o 
o 

9 
// 

5 
4.1 

12 
70.5 
O 
O 

24 

'7 

ii 
8.1 
6 
6 
14 

12  5 

15 
77.5 

13 

77 

2 

6< 

J-5 

0 
0 

'7 
'7 

34 

32.5 
H 

12 
0 
O 
0 

o 
o 

0 

8 

77 

10 
'* 

6-5 

27 

J4 

GH(W.B)x  W[BH]    ... 

GH(B  XW[BH] 

BHxVV[GB-H]  

BHxWfB-Iin  

BI(W.H)XW[GB-IH]  .... 
BI(W.H)XW[B-IH]  
BH(VV)xW[GHJ  

BH(W)XW[B-IH]   

12 
12.2 

40 
46 

56 

507 

84 
7' 

98 

770 

Totals   expected  

TABLE  4. — Average  grade  (size  of  stripe}  in  the  various  lots  of  hooded  rats  of  selected 
parentage,  as  compared  u>ith  the  average  grade  of  their  parents. 


Deviation 

Lot. 

No. 

Individuals. 

Average 
grade. 

Average 
grade 
of  parents. 

of  average 
filial  from 
average  pa- 

rental grade. 

E 

132 

211 

21.8 

-0.7 

F 

89 

12.9 

17.8 

-49 

G 

83 

96 

II.O 

-i  4 

H 

61 

5-6 

7-8 

—  2.2 

I 

34 

4.0 

+2.3 

J 

in 

15-8 

12.0 

+3-8 

K 

70 

22.2 

23.0 

-0.8 

L 

64 

36.2 

40.7 

-4-5 

If 

19 

524 

500 

+2-4 

IN   RATS   AND  GUINEA-PIGS. 


37 


5. — Relation  between  distribution  of  pigment  spots  in  i  ij  guinea-pigs  of 
group  D%,  and  in  their  parents. 


Offspring. 

Parents. 

Location  of 
spots  and  total 
number  of 
cases. 

4  spots. 

3  spots. 

2    SpOtS. 

i  spot. 

o  spots. 

Cases. 

P.  ct. 

Cases. 

P.  ct. 

Cases. 

P.  ct. 

Cases. 

P.  ct. 

Cases. 

P.  ct. 

2    Spots. 

i  spot., 
o  spots. 

Total.. 

2    SpOlS  . 

i  spot., 
o  spots. 

Total  . 
2  spots. 

4I 

I 

85.7 

12.3 

2 

10 
8 

55.6 

44-4 

35 
15 

70 
30 

1  Head  ; 
^  total  cases, 

J.,7. 

^ 
1  Shoulder; 
}•  total  cases, 
1  "7- 

|  Side  ; 
V  total  cases, 
1  "7- 

j  Rump  ; 
}  total  cases, 
1  "7- 
J 

49 

... 

IS 

... 

50 

4 

i 

80 
20 

3 
3 

2 

37-5 
37-5 
25 

8 

4 
10 

36-4 

18.2 
45-4 

... 

... 

13 
21 

43 

1.59 
25.6 

53.5 

5 

... 

8 

... 

22 

"... 

... 

... 

82 

7 

70 

3 

100 

16 
10 
18 

36-4 

22.7 
40.9 

3 

i 

75 
25 

'4 

3 

25 
25 
50 

o  spots. 
Total.. 

2    Spots. 

i  spot.. 
o  spots. 

Total.  . 

3 

3» 

... 

... 

10 

... 

3 

... 

44 

... 

4 

... 

5<> 

... 

25 
6 

4 

71.4 
17.2 
II-4 

... 

... 

4i 
20 

21 

5° 
24.4 
25.6 

... 

... 

... 

... 

35 

... 

... 

... 

82 

TABLE  6. — Combination  of  the  four  parts  of  table  5,  showing  in  general  the  extent  to  which 
pigment  spots  are  inherited  in  group  D±,  i  ij  individuals  (468  observations). 


Offspring. 

Parents. 

Total 

iiuiiib.-r 
of 
cases. 

U68 
J 

4  spots. 

3  spots. 

2    SpOtS. 

i  spot. 

o  spots. 

Cases. 

P.  ct. 

Cases. 

P.  ct. 

Cases. 

P.  ct. 

Cases. 

;P.  ct. 

Cases. 

P.  ct. 

2    SpOtS  .. 
I    Spot    ... 

o  spots... 
Total  .. 

78 

78.8 
132 

So 

16 

ii 

2 

55-2 
37-9 
69 

100 

49 
49 

50-5 
24.7 
24.7 

3 

... 

35 

19.6 

254 

55-o 

99 

29 

... 

198 

... 

4 

138    j     ... 

INHERITANCE  OF  COAT-PIGMENTS  AND  COAT-PATTERNS 


TABLE  7. — Relation  between  distribution  of  pigment  spots  in  30  guinea-pigs  of 
group  Di,  and  in  their  parents. 


Offspring. 

Parents. 

Location  of 
spots  and  total 
number  of 
cases. 

4  spots. 

3  spots. 

2    SpOtS. 

i   spot. 

o  spots. 

Cases. 

P.  ct. 

Cases. 

P.  ct. 

Cases. 

P.  ct. 

Cases. 

P.  ct. 

Cases. 

P.  ct. 

2    spots. 
I    spot.  . 

o  spots. 
Total.. 

2    Spots. 

i  spot., 
o  spots. 

Total.. 

2    Spots. 

I   spot.  . 

23 

5 

82.1 
17.9 

I 

|  Head  ; 
}•  total  cases, 
130. 

|  Shoulder; 
}•  total  cases, 
130. 

|  Side; 
}•  total  cases, 
130. 

|  Rump; 
}•  total  cases, 

130. 
J 





28 

2 

... 

... 

2 
2 
I 

I 

4 

5 
13 

18.3 

22.7 

59-° 

... 

... 

... 

I 
I 

... 





... 

5 

... 

... 

3 

22 

... 

2 
2 

... 

... 

... 

13 

4 
9 

50  o 
J5-3 
347 

o  spots. 
Total.. 

4 

... 

... 

... 

26 

... 

2    spots. 

i  spot.  . 
o  spots. 

Total.. 

24 
3 
3 

80 

IO 
10 









30 

TABLE  8. — Combination  of  the  four  parts  of  table  7,  showing  spot  inheritance  in 
group  DI,  30  individuals  (120  observations). 


Offspring. 

Parents. 

Total 
number 
of 
cases. 

4  spots. 

3  spots. 

2    SpOtS. 

i  spot. 

o  spots. 

Cases. 

P.  ct. 

Cases. 

P.  ct. 

Cases. 

P.  ct. 

Cases. 

P.  ct. 

Cases. 

P.  ct. 

2  Spots... 

i  spot.... 
o  spots... 

Total.... 

1 
3 

Si.o 
13-8 

S-2 

3 
3 

i 

... 

2 
2 

... 

I 
I 

... 

17 
9 

22 

~ 

45^8 

1 

}>I2O 
1 

J 

58 

... 

7 

... 

4 

... 

3 

... 

... 

IN  RATS  AND  GUINEA-PIGS. 


39 


9. — Relation  between  distribution  of  pigment  spots  in  ij8  guinea-pigs  of 
group  D2,  and  in  their  parents. 


Offspring. 

Parents. 

Location  of 
spots  and  total 
numbrr  of 
cases. 

4  spots. 

3  spots. 

2    SpOtS. 

i  spot. 

o  spots. 

Cases. 

P.  ct. 

Cases. 

P.  ct. 

Cases. 

P.  ct. 

Cases. 

P.  ct. 

Cases. 

P.  ct. 

2    Spots. 

i  spot.. 
o  spots. 

Total  . 

2    Spots. 

I   spot.. 
o  spots. 

Total  . 

2    Spots. 

i   spot., 
o  spots. 

Total. 

2    Spots. 

i   spot., 
o  spots. 

Total. 

154 

24 

86.5 

13.5 

|Head; 
^  total  cases, 
I  178. 

1 
|  Shoulder; 
}•  total  cases, 

h 

|  Side; 
}•  total  cases, 
|i78. 

|  Rump; 
V  total  cases, 
(178. 

178 

... 

... 

3 

i 
i 

•• 

5 
7 
7 

26.3 
36.8 
368 

7 
4 
4 

46.7 
26.7 
26.7 

49 
33 

57 

353 
23-7 
41. 

... 

5 

... 

'9 

... 

15 

... 

i39 

... 

6 

2 

3 

54-5 
18.2 

27-3 

4 

2 

... 

22 

7 

12 

53-6 
17.1 

29-3 

5 
i 
6 

41.7 

8-3 
50- 

39 

18 

5i 
108 

36-1 
16.7 
47.2 

ii 

... 

6 

... 

41 

... 

12 

... 

144 
16 

18 

80.9 
9.0 

10.  1 

178 

TABLE  10. — Combination  of  the  jour  parts  of  table  p,  showing  spot  inheritance  in 
group  D2,  178  individuals  (j  12  observations). 


Offspring. 

Parents. 

Total 
number 
of 
cases. 

4  spots. 

3  spots. 

2    SpOtS. 

i  spot. 

o  spots. 

Cases. 

P.  ct. 

Cases. 

P.  ct. 

Cases. 

P.  ct. 

Cases. 

P.  ct. 

Cases. 

P.  ct. 

2  spots... 

i  spot.  .. 
o  spots... 

Total... 

304 
42 
21 

82.8 
II.4 

5-7 

7 

3 

i 

63.6 

27.3 
9.1 

27 

H 

19 

45.0 
233 
31.7 

12 

5 
10 

44-4 
18.5 
37-0 

88 

& 

35-6 

20-7 

43-7 

712 

367 

... 

ii 

... 

60 

... 

27 

... 

247 

... 

INHERITANCE  OF  COAT-PIGMENTS  AND  COAT-PATTERNS 


TABLE  u. — Relation  between  distribution  of  pigment  spots  in  136  guinea-pigs  of 
group  D3,  and  in  their  parents. 


Offspring. 

Parents. 

Location  of 
spots  and  total 
number  of 
cases. 

4  spots. 

3  spots. 

2    Spots. 

i  spot. 

o  spots. 

Cases. 

P.  ct. 

Cases. 

P.  ct. 

Cases. 

P.  ct. 

Cases. 

P.  Ct. 

Cases. 

P.  ct. 

2    Spots. 
I    Spot.. 

o  spots. 
Total. 

2    Spots. 

I  spot.  . 
o  spots. 

Total. 

2    Spots. 

I   spot., 
o  spots. 

Total  . 

2    Spots. 

i  spot.  . 
o  spots. 

Total. 

117 
H 

89.3 

10.7 

4 

I 

80 

20 

|  Head  ; 
)>  total  cases, 

!  136- 

|  Shoulder; 
}•  total  cases, 
1  136. 

|  Side; 
}•  total  cases, 
1  136. 

1 
1  Rump; 
\  total  cases. 
|i36- 

— 

131 

... 

5 

.. 

... 

2 

... 

... 

< 

... 

48 

28 

55 

36.6 
21.4 
42.0 

' 

... 

... 

... 

3 

2 

«3« 

... 

2 
2 
2 

33- 
33- 
33- 

... 

... 

38 

H 

20 

52-8 
194 

27.8 

... 

... 

'9 
13 
26 

32.8 

22.4 

44.8 

6 

... 

... 

... 

72 

... 

... 

... 

58 

... 

"3 
H 
9 

83-1 
10.3 
6.6 

136 

TABLE  12. — Combination  of  the  four  parts  of  table  //,  showing  spot  inheritance  in 
group  D$,  1 36  individuals  (544  observations). 


Parents. 

Total 

Offspring. 

4  spots. 

3  spots. 

3    SpOtS. 

i  spot. 

o  spots. 

number 
of 

Cases. 

P.  ct. 

Cases. 

P.  ct. 

Cases. 

P.  ct. 

Cases. 

P.  ct. 

Cases. 

P.  ct. 

2    Spots... 

232 

85.0 

6 

75 

38 

52.8 

, 

... 

67 

35-5 

1 

i  spot.... 
o  spots... 

30 
II 

II.  O 
4.0 

2 

25 

H 
20 

19.4 

27.8 

..! 

... 

48; 

21.7 
42.8 

!>544 

Total.... 

273 

... 

8 

... 

72 

... 

2 

... 

i89 

... 

J 

IN  RATS  AND  GUINEA-PIGS.  41 

TABLE  13. — Combinations  separately  of  the    four  parts  of  tables  5,  7,  9,  and  //,  showing 
inheritance  of  particular  spots  in  series  D  as  a  whole,  461  individuals. 


Off  sping. 

Parents. 

Location  of 
spots  and  total 
number  of 
cases. 

4  spots. 

3  spots. 

2    SpOtS. 

i  spot. 

o  spots. 

Cases. 

P.  ct. 

Cases. 

P.  ct. 

Cases. 

P.  ct. 

Cases. 

P.  ct. 

Cases. 

P.  ct. 

2    Spots. 

i  spot., 
o  spots. 

Total.. 

2    Spots. 

I  spot., 
o  spots. 

Total.. 

2    Spots 

i  spot.  . 
o  spots. 

Total.. 

2    SpOtS. 

I  spot.  . 
o  spots. 

Total  . 

336 

49 

87.0 
12.7 
o-3 

*5 
10 

60 

40 

35 
15 

70 

30 

|  Head; 
[•total  cases, 
1  461. 

j  Shoulder; 
)>  total  cases, 
|46i. 

|Side; 
}•  total  cases, 
|46i. 

|  Rump; 
}•  total  cases, 
!  461. 

J 

386 

... 

25 

... 

... 

4 

i 

80 

20 

10 

7 
4 

21 

47-6 
33-3 

KJ.O 

13 
ii 

17 

£1 

4i-5 

9 

6 

450 
25.0 
30.0 

114 
87 
173 

30-5 
23.2 

46.3 

5 

... 

... 

4i 

... 

20 

374 

... 

15 
8 

SS-6 
I4.8 
29.6 

7 

2 

77-8 
22.2 

78 
33 
50 

48.4 
20.5 
41.0 

8 

2 

6 

50.0 
12-5 
37-5 

85 
49 
114 

34-3 
19.7 
46.0 

27 

... 

9 

... 

161 

... 

16 

... 

248 

... 

306 
39 
34 

80.7 
10-3 
9.0 

... 

... 

41 
20 

21 

500 
24.4 
25.6 

379 

... 

... 

... 

82 

TABLE  14. — Combination  of  tables  6,  8,  10,  and   12,   showing  spot  inheritance  in 
series  D  in  general,  461  individuals  (1,844  observations}. 


Offspring. 

Parents. 

Total 
number 
of 
cases. 

4  spots. 

3  spots. 

2    SpOtS. 

i  spot. 

o  spots. 

Cases. 

P.  ct. 

Cases. 

P.  ct. 

Cases. 

P.  ct. 

Cases. 

P.  ct. 

Cases. 

P.  ct. 

2  SpOtS.. 

i  spot.... 
o  spots... 

Total... 

661 
93 
43 

82.9 
11.7 

5-4 

32 
19 
4 

58.2 

345 
7-3 

167 
i 

334 

50.0 

237 
26.3 

'1 

II 

47.2 
22.2 
30.6 

199 

32.0 
21.9 
46.1 

^844 

797 

... 

55 

... 

... 

36 

... 

622 

... 

INHERITANCE  OF   COAT-PIGMENTS  AND  COAT-PATTERNS 


TABLE  15. — Relation  between  distribution  of  pigment  spots  in  207  guinea-pigs  of 
group  H±,  and  in  their  parents. 


Offspring. 

Parents. 

Location  of 
spots  and  total 
number  of 
cases. 

4  spots. 

3  spots. 

2    SpOtS. 

i  spot. 

o  spots. 

Cases. 

P.  ct. 

Cases. 

P.  ct. 

Cases. 

P.  ct. 

Cases. 

P.ct. 

Cases 

P.  ct. 

2    spots. 
I    Spot.. 

o  spots. 
Total  . 

2    Spots. 

i  spot.  . 

8.3 

12 

9 

79.8 

"-S 

8.7 

20 
13 

33 

60.6 

394 

35 
15 

2 

673 

28.8 

38 

10 

3 

76.9 

23  » 

3 

2 

60. 
40. 

]  Head  ; 
}  total  cases, 
I  207. 

1 
|  Shoulder; 
}  total  cases, 
i  207. 

J 

1  Side  ; 
}  total  cases, 

1  207. 

|  Rump; 
V  total  cases, 
1  207. 

104 

... 

... 

52 

... 

13 

5 

... 

H 

7 

21 

33-3 
16.7 
50.0 

... 

... 

27 

£ 

16.4 
194 
64.2 

o  spots. 
Total. 

2    Spots. 

i   spot.. 

42 

i6c 

20 

9 

21 

40.0 
18.0 
420 

i 

5 
5 

9.1 

45-4 
45-4 

25 
3i 
90 

17.1 

21.2 
6l.7 

o  spots. 
Total  . 

2    SpOtS. 

i  spot  .  . 

50 

... 

ii 

146 

... 

44 
17 
23 

52-4 

2O  2 

27-4 

10 

8 
ii 

34-5 
27.6 

379 

26 
14 
54 

27.7 
I4-9 

57-4 

o  spots. 
Total. 

84 

... 

29 

... 

94 

... 

TABLE  16. — Combination  of  the  four  parts  of  table  15,  showing  spot  inheritance  in 
group  H\t  207  individuals  (828  observations}. 


Offspring. 

Parents. 

Total 
number 
of 
cases. 

4  spots. 

3  spots. 

2    SpOtS. 

i  spot. 

o  spots. 

Cases.  1   P.  ct. 

Cases. 

P.ct. 

Cases. 

P.  ct. 

Cases. 

P.  ct. 

Cases. 

P.ct. 

2   spots... 
I    spot  

o  spots... 
Total.... 

83 
12 

9 

79-8 
n-5 
8.7 

20 
13 

60.6 

39-4 

"3 

| 

49.6 
21.  0 
29.4 

21 

16 
16 

39  6 
30.2 
30.2 

81 

77 
252 

Ill 

61.5 

1 
^828 

104 

... 

33 

... 

228 

... 

53 

... 

410 

... 

IN   RATS  AND  GUINEA-PIGS. 

TABLE  17. — Relation  between  distribution  of  pigment  spots  in  193  guinea-pigs  of 
group  Hi,  and  in  their  parents. 


43 


Offspring. 

Parents. 

Location  of 
spots  and  total 
number  of 
cases. 

4  spots. 

3  spots. 

2    SpOtS. 

i   spot. 

o  spots. 

Cases. 

P.  ct. 

Cases. 

P.  ct. 

Cases. 

P.  ct. 

Cases. 

P.ct. 

Cases. 

P.  ct. 

2    spots. 

I  spot.  . 
o  spots. 

Total  . 

2    Spots. 

i  spot.  . 

88 
II 
4 

854 
10.7 

3-9 

13 

5 

68.4 
26.3 

5-3 

37 
21 

7 

57-o 

32-3 
10.7 

3 

i 

2 

6 

50.0 

16.7 

333 

... 

... 

1  Head  ; 
[-total  cases, 
1  193- 

j  Shoulder; 
}  total  cases, 
1  193- 

Side  ; 
\  total  cases, 
193. 

|  Rump; 
V  total  cases, 
i  193- 

103 

... 

19 

... 

65 

... 

... 

... 

... 

14 
140 

39 

7.3 

20.7 
72.0 

o  spots. 
Total. 

2    Spots. 

i  spot.. 

*93 

... 

29 

35 
129 

15.0 
66.8 

o  spots. 
Total. 

2    SpOtS. 

i  spot.. 

54 

& 

28.0 
16.0 
56.0 

o  spots. 
Total. 



; 

193 

... 

1 8. — Combination  of  the  jour  parts  of  table  //,  showing  spot  inheritance 
in  group  HI,  193  individuals  (772  observations). 


Offspring. 

Parents. 

Total 
number 
of 
cases. 

4  spots. 

3  spots. 

2    SpOtS. 

i  spot. 

o  spots. 

Cases. 

P.  ct. 

Cases. 

P.  ct. 

Cases. 

P.  ct. 

Cases. 

P.  ct. 

Cases. 

P.  ct. 

2  Spots.. 

i  spot.  ... 
o  spots... 

Total... 

88 
ii 

4 

85.4 
10.7 

3-9 

13 

5 
i 

684 

26.3 

5-3 

37 
21 

7 

56.9 
323 

10.8 

3 
i 

2 

50 
16.7 

33-3 

97 
1  06 
376 

16.8 

i8'3 

64.9 

[772 
J 

103 

19 

... 

65 

... 

6 

... 

579 

44 


INHERITANCE  OK   COAT-PIGMENTS   AND  COAT-PATTERNS 


TABI,E  19. — Relation  between  distribution  of  pigment  spots  in  1 29  guinea-pigs  of 
group  7/2,  and  in  their  parents. 


Offspring. 

Parents. 

Location  of 
spots  and  total 
number  of 
cases. 

4  spots. 

3  spots. 

2    SpOtS. 

i   spot. 

o  spots. 

Cases. 

P.  ct. 

Cases. 

P.  ct. 

Cases. 

P.  ct. 

Ca^es. 

P.  ct. 

Cases 

P.  ct. 

2    spots, 
i  spot., 
o  spots. 

Total.. 

2    SpOtS. 

i  spot.. 
o  spots. 

Total.  . 

2    SpOtS. 

i  spot., 
o  spots. 

Total.. 

2    Spots. 

i  spot., 
o  spots. 

Total  .  . 

74 

20 

2 

96 

77.J 
20.8 
2.1 

10 

2 

83-3 
16.7 

*7 

4 

81.0 
190 

|  Head; 
}•  total  cases, 
1  129. 

|  Shoulder; 
}•  total  cases, 
1  129. 

JSide; 
total  cases, 
129. 

1 
I  Rump; 
}•  total  cases, 
1  129. 

... 

12 

... 

21 

4 

99 

3-i 

20  2 

76.7 



129 

... 

ii 

3° 

88 

8-5 
23-3 

68.2 

129 

... 

36 

J2 

27.9 
58-9 





129 

... 

TAEI,S  20. — Combination  of  the  four  parts  of  table  19,  showing  spot  inheritance  in 
group  //2,  129  individuals  (5  /  6  observations) . 


Offspring. 

Parents. 

Total 
number 
of 

cases. 

4  spots. 

3  spots. 

2    SpOtS. 

i  spot. 

o  spots. 

Cases. 

P.  ct. 

Cases. 

P.  ct. 

Cases. 

P.  ct. 

Cases. 

P.  ct. 

P.  ct. 

P.  ct. 

2  Spots... 

i  spot  — 
o  spots... 

Total..  . 

74 
2O 

2 

771 
20.8 
2.1 

10 

2 

83-3 
16.7 

17 

4 

81.0 
19.0 

... 

... 

51 

% 

13.2 

18.8 
68.0 

^516 

96 

... 

12 

... 

21 

... 

... 

... 

387 

... 

IN   RATS  AND  GUINEA-PIGS. 

TABLE  21. — Relation  between  distribution  of  pigment  spots  in  $8  guinea-pigs  of 
group  //3,  and  in  their  parents. 


45 


Offspring. 

Parents. 

Location  of 
spots  and  total 
number  of 
cases. 

4  spots. 

3  spots. 

2    SpOtS. 

i  spot. 

o  spots. 

Cases. 

P.  ct. 

Oses. 

P.  ct. 

Cases. 

P.  ct. 

Cases. 

P.  ct. 

Cases. 

P.  ct. 

2  Spots.  . 
I  Spot  .  . 

46 
10 

82.1 
17.9 

I 
I 

1  Head  ; 
J  total  cases, 
» 

Shoulder; 
>  total  cases. 
,58- 

1  Side; 
J  total  cases, 
5, 

1  Rump; 
>  total  cases, 
,58. 

Total.. 

2  Spots.  . 

56 

... 

2 

2 

9 
47 

34 
'5-5 
81.1 

i  spot  .  . 

o  spots.. 

Total  .  . 

58 

... 

2  spots.. 

4 
H 
40 

58 

6.9 
24.1 
69.0 

i  spot  .  . 
o  spots.. 

Total.. 

2    Spots  . 

i  spot.  . 
o  spots  . 

Total  .  . 



... 

*3 
37 

22.4 
13.8 
63.8 

58 

... 

TABLE  22. — Combination  of  the  four  parts  of  table  21,  showing  spot  inheritance  in 
group  //3,  58  individuals  (232  observations). 


Offspring. 

Parents. 

Total 
number 
of 
cases. 

4  spots. 

3  spots. 

2    SpOtS. 

i   spot. 

o  spots. 

Cases. 

P.  ct. 

Cases. 

P.  ct. 

Cases. 

P.  ct. 

Cases. 

P.  ct. 

Cases. 

P.  ct. 

2    Spots... 

i  spot.... 

46 
10 

82.1 
17.9 

I 

19 
31 
I24 

10.9 
17.8 
71-3 

232 

Total... 

56 

... 

* 

... 

174 

... 

46 


INHERITANCE  OF  COAT-PIGMENTS  AND  COAT-PATTERNS 


TABLE  23. — Combinations  separately  of  the  four  parts  of  tables  15,  ij,  19,  and  21, 
showing  inheritance  of  particular  spots  in  series  H  as  a  whole,  587  individuals. 


Offspring. 

Parents. 

Location  of 
spots  and  total 
number  of 
cases. 

4  spots. 

3  spots. 

2    SpOtS. 

i  spot. 

o  spots. 

Cases. 

P.  ct. 

Cases. 

P.  ct. 

Cases. 

P.  ct. 

Cases. 

P.  ct. 

Cases. 

P.  ct. 

2    Spots. 
I    SpOt.. 

o  spots. 
Total. 

2    SpOtS. 

291 

53 
15 

81.1 
14.7 
4.2 

44 
21 

I 

66.7 
31.8 

i-5 

89 
40 

9 

64.5 
29.0 

6-5 

13 

4 

2 

68.4 

21.  1 
10.5 

3 

2 

60. 
40. 

IHead  ; 
total  cases, 
587. 

1  Shoulder; 
}•  total  cases, 

1587. 

ISide; 
}  total  cases, 
1  587. 

IRump; 
total  cases, 
587. 

359 

... 

66 

... 

138 

... 

19 

... 

5 

... 

H 

7 

21 

33-3 
16.7 

50. 

... 

... 

47 
107 

39i 

18.6 
19.7 
71.7 

o  spots. 
Total  . 

2    spots. 

i  spot.  . 



42 

... 

... 

... 

545 

... 

20 

9 

21 

40. 
18. 
42. 

I 

5 
5 

9.I 

45-4 
45-4 

69 
no 

347 

13-1 
20.9 
66.0 

o  spots. 
Total. 

2    Spots. 



50 

... 

ii 

... 

526 

... 

44 
17 
23 

52-4 

20.2 
27.4 

10 

8 
ii 

34-5 
27.6 

37-9 

129 

70 
275 

27.2 
14.8 
58.0 

o  spots. 
Total  . 

84 

... 

29 

... 

474 

... 

TABLE  24. — Combination  of  tables  16,  18,  20,  and  22,  showing  spot  inheritance  in 
series  H  in  general,  587  individuals  (2348  observations). 


Offspring. 

Parents. 

Total 
number 
of 
cases. 

4  spots. 

3  spots. 

2   SpOtS. 

i  spot. 

o  spots. 

Cases. 

P.  ct. 

Cases. 

P.  ct. 

Cases. 

P.  ct. 

Cases. 

P.  ct. 

Cases. 

P.  ct. 

2  spots... 

i  spot  — 
o  spots... 

Total... 

291 
53 
15 

81.1 
14.7 

4.2 

44 
21 

I 

66.7 
31.8 

i-5 

I67 

73 

74 

53-2 
23-3 
23-5 

24 
17 

40.7 
288 
30-5 

248 
287 
1015 

16.0 
18.5 
65-5 

1 
^348 

359 

... 

66 

... 

3H 

... 

59 

... 

J55° 

TABLE  25. — Frequencies  with  which  spots  absent  in  the  parents  (or  in  one  parent, 
in  group  Hi)  are  absent  also  in  their  offspring. 


Group  Hi 
(table  15). 

Group  Hi 

(table  17). 

Group  Ha 

(table  19). 

Group  HZ 
(table  21). 

Average 
(table  23). 

Shoulder  spots  
Side  spots  

TVr  ct. 
64.2 
61.7 

Psr  ct. 
72  o 
66.8 

Per  ct. 

76.7 
68.2 

P<*rct. 
8l.I 
69.0 

Per  ct. 

71.7 
66.0 

Rump  spots  

57-4 

56  o 

58.9 

638 

58.0 

IN  RATS  AND  GUINEA-PIGS. 


47 


TABLE  26  — Constants  for  the  four  groups  of  series  D,  based  upon  tables  6,  8,  /o, 

and   12,  respectively. 


Group  D£. 

Group  Di. 

Group  Da. 

Group  Dj. 

Average  number  of  spots  borne 
by  a  parent           .  . 

3  78 

44.O 

A    62 

A  f>± 

Average  number  of  spots  borne 

4-.  76 

C.44 

T-  "f 

c.c6 

4.LKJ. 

5*72 

Per  cent  increase  over  parents  .... 
Standard  deviation  (<y)  in  num- 
ber of  spots,  parents  

26 
2.876 

24 

3  812 

20 

•5  744 

»* 

23 

7  6o<l 

Standard  deviation  (<y)  in  num- 
ber of  spots    offspring. 

3  CO  7 

•j  6ic 

•5    663 

O   UVT 
?   662 

Coefficient    of  variability,    par- 
ents      

J^J 

76.78 

ge  12 

80  QO 

Coefficient    of   variability,    off- 

72.02 

67.70 

6c.7o 

/y  T~f 

64  20 

Coefficient    of   correlation    be- 
tween parents  and  offspring.  . 

0.4527 

0.4580 

0.4278 

°-4343 

TABLE  27. — Constants  for  tlie  four  groups  of  individuals  of  series  Ht  based  upon 
tables  1 6,  18,  20,  and  22,  respectively. 


Group  HJ. 

Group  Hi. 

Group  H?. 

Group   H3. 

Average  number  of  spots  borne 
by  a  parent  .              

2  4.8 

I.C6 

I  80 

2  OO 

Average  number  of  spots  borne 
by  offspring  

T  88 

•j  20 

^oS 

•?  oo 

Per  cent  increase  over  parents  
Standard  deviation  (^-)  in  num- 
ber of  spots    parents  

56 

2  870 

105 
'y  022 

71 
1  ing 

50 
34.74. 

Standard  deviation  (<r)  in  num- 

•j  c76S 

•?  6O76 

•?.tq6o 

•'t^T 

3  6200 

Coefficient    of   variability,  par- 
ents                     

1IC  72 

lS6  C7 

177  66 

I7l  *7O 

Coefficient    of    variability,    off- 
sprino1..., 

Q2   iS 

1  12  7C 

11^  OO 

•/••/** 

1  2O  60 

Coefficient    of    correlation    be- 
tween parents  and  offspring.... 

°4565 

0.6076 

o  6109 

06796 

TABLE  28.  —  Character  of  the  young  of  the  original  nose-spot  male, 


(NSo). 


Individuals 
with 
nose  spots. 

Z 
$* 

3& 

r 

Average 
number 
other  spots. 

Individuals 
without 
nose  spots. 

jj 

"*  % 
1 

III 
il] 

<  B-S 
0 

Per  cent  of 
young  with 
nose  spots. 

By  mothers  without  nose  spots 
By  NS0  mothers  

4 

22 

6 

58 

1-5 

2.6 

H 
19 

40 

58 

2.8 

3.0 

22.2 

S3  6 

By  NSi  mothers     

16 

•2Q 

2.4 

io 

2Q 

2.9 

61  «: 

? 

C 

1.6 

2 

i 

o.  c 

w«O 

60.0 

Total  

4C 

108 

2.4 

4C 

128 

2.8 

CO.O 

48  INHERITANCE  OF  COAT-PIGMENTS  AND  COAT-PATTERNS 

29  — Cliaracter  of  the  young  of  nose-spot  males,  chiefly  descendants  of  19890  (table  28}, 


•3   § 

1 

« 

•3«  a 

i 

i 

M'j 

-ajs  a, 

o  * 

rt*J)  §, 

||| 

o  15 

1t£  §• 

c  ^  §. 

'>•!  u 

li 

||| 

•38. 

>  1  S 

"  c"w 

1  I 

0 

^•5  s 

0   * 

•<  c-g 

*"   3   o 

H 

<:C| 

5        § 

H 

o 

(2  >^s 

c?5595«  NSo     by  NS0  mothers 

x 

5 

5- 

6 

27 

45 

H-3 

NSi 
6^5678,  NSi           NSo 

4 

0 

10 

o 

o. 

5 

2 

15 

5 

3- 

25 

44-4 
o. 

NSX 

2 

6 

3- 

3 

8 

2-7 

40. 

6^5151,  NS2          NS0 

15 

30 

2. 

22 

66 

3- 

4°-5 

44         44            NSi 

9 

32 

3-5 

13 

52 

4- 

4°  9 

^5652,  NS2          NS0 

7 

16 

2-3 

5 

13 

2.6 

583 

NS2 

2 

3 

2 

9 

4.5 

c?5388,  NS,*,        NSo 

0 

o 

0. 

4 

14 

3-5 

0. 

NSi 

5 

6 

1.2 

6 

ii 

1.8 

45-4 

(^5669,  NS2*,        NSo 

3 

3- 

4 

10 

2-5 

20. 

44  ,        NSz 

2 

5 

2-5 

i 

i 

i. 

67. 

"           "  ,        NS2 

I 

5 

5- 

i 

4 

4- 

50- 

Total  

40 

121 

2.5 

74 

275 

1.2 

39.8 

1^  7 

O 

/  T 

oo 

•j 

TABLE  30. — Further  analysis  of  the  data  contained  in  table  29. 


•3    a 

11! 

it 

«*.£  £• 
Sfltt 

>   3  £ 

|if 

-£      . 

«"B  * 

>   3   5 

lit 
"?J 

^    ?    o 

*^     P  rG 

V    ^    O 

i—  i         C 

H 

0 

>S      c 

H 

O 

».Ra 

NSo  males  by  NS0  females  

I 

5 

5-° 

8 

32 

40 

ii 

NSo  males  by  NSi  females  

6 

16 

2.7 

8 

23 

2-9 

43 

NSi  or  1^  males  by  NSo  females. 

23 

49 

2.1 

35 

103 

2.9 

396 

NSi  or,*malesbyNSi  females. 

16 

43 

2.7 

20 

64 

3-2 

44 

NSi  or  i^.  males  by  NSi  females. 

3 

8 

2  7 

3 

13 

4-3 

50 

Totals  for  NS0  males  

7 

21 

3.0 

16 

55 

34 

3°  4 

Totals  for  NSi  or  Tj  males... 

42 

100 

2.2 

58 

180 

42 

Totals  for  NS0  females 

24 

54 

2.1 

43 

IIS 

-  i 

35-8 

Totals  for  NSi  females  

22 

59 

2  7 

28 

87 

3.1 

44 

Totals  for  NS2  females  

3 

8 

27 

3 

13 

43 

50 

Grand  totals     

4O 

121 

2.5 

74 

275 

7.2 

39-8 

IN   RATS  AND  GUINEA-PIGS. 


49 


31. — Character  as  regards  nose  spots  of  the  young  of  animals  of  Series  H 
(head  spot  series).     None  of  the  parents  bore  nose  spots. 


Father. 

°f 
H 

Spots  of 
mother 
(average). 

Spots,  of 
parents 
(average). 

Individuals 
with 
nose  spots. 

$S 

r 

Average 
number 
other  spots. 

Individuals 
without 
nose-spots. 

Total  other 
spots. 

Average 
number 
other  spots. 

Per  cent  of 
young  with 
nose  spots. 

1716* 

I 

2.O 

•5 

4 

16 

4.0 

29 

100 

3-4 

12.  I 

2537 

0 

2-5 

•*5 

10 

19 

1.9 

63 

197 

3-1 

38.7 

2625 
438S 

I 

0 

1.9 

3-2 

:i 

H 

4 

32 

7 

2.3 
i-7 

47 
34 

132 
92 

2.8 
2.8 

23 
10.5 

4399 

I 

2.7 

.8 

7 

M 

2 

21 

48 

2.3 

25 

5420 

I 

i-7 

•3 

... 

... 

6 

18 

3-o 

O 

Total  . 

0.56 

2-3 

14 

39 

88 

2.3 

200 

589 

2.9 

16.3 

*By  an  NS0  female,  this  same  male  (1716)  had  ten  young,  three  of  which  bore  nose  spots.     The  average  number 
of  spots  other  than  nose  spots  borne  by  ihe  NS  young  was  2.6;  by  the  remaining  young,  3.7. 

Data  for  curves  (figs.  2  to  5). 


Classes... 

4-5 

'4-5 

24-5 

34-5 

44-5 

5^-J 

64.5 

74-5 

Average. 

A 

h  r 

64. 

•tc 

11  "t 

B  

2Q 

7C 

7,4 

1C 

6 

7 

A 

L6-6 
21.2 

c  

8 

7 

9 

2 

i 

27  C 

D 

4? 

C7 

2Q 

9 

IT 

17 

I  I 

i 

27  Q 

E  
p 

24 

•32 

48 

42 

29 

12 

16 

IO 

5 

21.  1 

T-7  Q 

**  y 

Classes... 

2 

7 

12 

'7 

22 

*7 

^ 

... 

... 

G 

28 

17 

\n 

12 

6 

2 

I 

Q  6c 

1{ 

•5  I 

1  / 
2O 

7 

2 

I 

y  wj 

r  A 

I  

Ol 

24. 

6 

Classes... 

4-5 

<4-5 

24-5 

345 

^•5 

54-5 

645 

7*.  5 

... 

7 

74. 

A  A 

2  1 

10 

j 

i 

1C  76 

£:..:.:" 

8 

TT 

18 

16 

i 

I 

22  2 

L  
M  

4 

9 

'4 
i 

10 

2 

9 

11 
6 

4 

c 

3 

36.2 
C2.4 

BIBLIOGRAPHY. 

,  G.  M. 
:  04.     The  heredity  of  coat-color  in  mice.     Proc.  Amer.  Acad.  Arts  and  Sci.,  vol.  40, 

No.  2,  pp.  61-163,  2  figs. 
BATESON,  W. 

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